Handle PBA that shares a page with MSI-X table for passthrough devices.

If the PBA shares a page with the MSI-X table, map the shared page via
/dev/mem and emulate accesses to the portion of the PBA in the shared
page by accessing the mapped page.

Reviewed by: grehan
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D5919