IH-747 - xapi/import: Use get_by_uuid_opt to not log backtraces when …

…failure is expected

Migration logs are always full of exceptions that are expected and immediately
handled:
```
[error|backtrace] SR.get_by_uuid D:8651cc0c9fb6 failed with exception Db_exn.Read_missing_uuid("SR", "", "a94bf103-0169-6d70-8874-334261f5098e")
[error|backtrace] Raised Db_exn.Read_missing_uuid("SR", "", "a94bf103-0169-6d70-8874-334261f5098e")
[error|backtrace] 1/9 xapi Raised at file ocaml/database/db_cache_impl.ml, line 237
[error|backtrace] 2/9 xapi Called from file ocaml/xapi/db_actions.ml, line 13309
[error|backtrace] 3/9 xapi Called from file ocaml/xapi/rbac.ml, line 188
[error|backtrace] 4/9 xapi Called from file ocaml/xapi/rbac.ml, line 197
[error|backtrace] 5/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 74
[error|backtrace] 6/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 96
[error|backtrace] 7/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
[error|backtrace] 8/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
[error|backtrace] 9/9 xapi Called from file ocaml/libs/log/debug.ml, line 250
[ warn|import] Failed to find SR with UUID: a94bf103-0169-6d70-8874-334261f5098e content-type: user - will still try to find individual VDIs
[....]
[debug|import] Importing 1 VM_guest_metrics(s)
[debug|import] Importing 1 VM_metrics(s)
[debug|import] Importing 1 VM(s)
[debug|import] Importing 1 network(s)
[debug|import] Importing 0 GPU_group(s)
[debug|import] Importing 1 VBD(s)
[error|backtrace] VBD.get_by_uuid D:3a12311e8be4 failed with exception Db_exn.Read_missing_uuid("VBD", "", "026d61e9-ed8a-fc72-7fd3-77422585baff")
[error|backtrace] Raised Db_exn.Read_missing_uuid("VBD", "", "026d61e9-ed8a-fc72-7fd3-77422585baff")
[error|backtrace] 1/9 xapi Raised at file ocaml/database/db_cache_impl.ml, line 237
[error|backtrace] 2/9 xapi Called from file ocaml/xapi/db_actions.ml, line 14485
[error|backtrace] 3/9 xapi Called from file ocaml/xapi/rbac.ml, line 188
[error|backtrace] 4/9 xapi Called from file ocaml/xapi/rbac.ml, line 197
[error|backtrace] 5/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 74
[error|backtrace] 6/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 96
[error|backtrace] 7/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
[error|backtrace] 8/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
[error|backtrace] 9/9 xapi Called from file ocaml/libs/log/debug.ml, line 250
[debug|import] Importing 1 VIF(s)
[error|backtrace] VIF.get_by_uuid D:2bc78449e0bc failed with exception Db_exn.Read_missing_uuid("VIF", "", "7d14aee4-47a4-e271-4f64-fe9f9ef6d50b")
[error|backtrace] Raised Db_exn.Read_missing_uuid("VIF", "", "7d14aee4-47a4-e271-4f64-fe9f9ef6d50b")
[error|backtrace] 1/9 xapi Raised at file ocaml/database/db_cache_impl.ml, line 237
[error|backtrace] 2/9 xapi Called from file ocaml/xapi/db_actions.ml, line 10813
[error|backtrace] 3/9 xapi Called from file ocaml/xapi/rbac.ml, line 188
[error|backtrace] 4/9 xapi Called from file ocaml/xapi/rbac.ml, line 197
[error|backtrace] 5/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 74
[error|backtrace] 6/9 xapi Called from file ocaml/xapi/server_helpers.ml, line 96
[error|backtrace] 7/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
[error|backtrace] 8/9 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
[error|backtrace] 9/9 xapi Called from file ocaml/libs/log/debug.ml, line 250
```

Use an internal get_by_uuid_opt call and match on the Option instead,
with the logs looking much clearer:
```
[debug|import] Importing 1 host(s)
[debug|import] Importing 2 SR(s)
[ warn|import] Failed to find SR with UUID: 8568e308-c61c-3b10-3953-45606cfecede content-type:  - will still try to find individual VDIs
[ warn|import] Failed to find SR with UUID: 40e9e252-46ac-ed3d-7a4c-6db175212195 content-type: user - will still try to find individual VDIs
[...]
[debug|import] Importing 2 VM_guest_metrics(s)
[debug|import] Importing 2 VM(s)
[debug|import] Importing 1 network(s)
[debug|import] Importing 1 GPU_group(s)
[debug|import] Importing 4 VBD(s)
[ info|import] Did not find an already existing VBD with the same uuid=569d0e60-6a89-d1fa-2ed6-38b8eebe9065, try to create a new one
[ info|import] Did not find an already existing VBD with the same uuid=533306da-cff1-7ada-71f7-2c4de8a0065b, try to create a new one
[ info|import] Did not find an already existing VBD with the same uuid=f9dec620-0180-f67f-6711-7f9e5222a682, try to create a new one
[ info|import] Did not find an already existing VBD with the same uuid=05e55076-b559-9b49-c247-e7850984ddae, try to create a new one
[debug|import] Importing 2 VIF(s)
[ info|import] Did not find an already existing VIF with the same uuid=a5a731d5-622c-5ca5-5b2a-a0053a11ef07, try to create a new one
[ info|import] Did not find an already existing VIF with the same uuid=1738bf20-8d16-0d69-48cd-8f3d9e7ea791, try to create a new one
```

Signed-off-by: Andrii Sultanov <[email protected]>