Cleanup PCI legacy interface section description

[paravmellanox]

Problem statement:
Legacy interface PCI Device layout description has following issues.

1. repeated 'structure' word
2. virtio header was defined the 0.9.5 spec. It is referred with
   different keywards in this section with multiple different words
   as (a) virtio header, (b) general headers, (c) legacy configuration
   structure, (d) virtio common configuration structure and
   (e) other fields.
3. Driver and device requirements listing is intermixed.
4. spelling error of structure
5. Legacy interface common configuration requirements are not adjacent
   to 1.x comm

Solution: Fix the above issues.
Patch: https://lists.oasis-open.org/archives/virtio-dev/202302/msg00603.html