virtio_pci_legacy: Use the table BAR and PBA BAR from MSI-X cap

The MSI-X resource shouldn't be assumed to be always on BAR1.
The Virtio v1.1 Spec did not specify that MSI-X table and PBA BAR has to
be BAR1 either.

Reported by: Yuan Rui <number201724@me.com>
MFC after: 2 weeks
Reviewed by: bryanv, jhb
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D28817