Skip determining unix socket if a remote config address is set

[MusicDin]

This PR prevents unix socket to be determined if remote config address is set.

When this PR is merged, users will be able to create an arbitrary default remote and use it with lxd-user socket.
For example:

provider "lxd" {
  lxd_remote {
    name     = "lxd-users"
    scheme   = "unix"
    address  = "/var/snap/lxd/common/lxd-user/unix.socket"
    default  = true
  }
}

resource "lxd_instance" "instance" {
  remote  = "lxd-users"
  project = "user-1001"

  name      = "tf-c1"
  image     = "images:alpine/3.16"
  ephemeral = false
}

^ Above currently does not work, because provider tries to determine the unix socket for LXD defined remotes (such as images remote).