Don't simulate PBA access if the PBA is in a separate BAR.

bhyve has to virtualize the MSI-X table to trap reads and writes to
that table and map those to virtual interrupts that it maps real host
interrupts on to. For the pending-bit-array (PBA), bhyve passes
accesses from the guest directly to the hardware.

bhyve's virtualization of the MSI-X table is done by intercepting all
reads and writes to the BAR holding the MSI-X table. However, if the
PBA is stored in the same BAR as the MSI-X table, accesses to the PBA
portion of this BAR have to be forwarded to the real BAR.

However, in the case that the PBA was stored in a separate BAR and
it's offset in that separate BAR overlapped with the portion of the
MSI-X table BAR that the table used, the handlers for the table BAR
would incorrectly think that some accesses were PBA reads and writes.
This caused a crash in bhyve when it indirected a NULL pointer. Fix
this case by never trying to handle PBA access if the PBA lives in a
separate BAR.

Reported by: gallatin
Tested by: gallatin
Reviewed by: markj, Patrick Mooney
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20523