Keep the shadow PCIR_COMMAND synced with the real one for pass through.

This ensures that bhyve properly recognizes when decoding is disabled
for BARs on passthru devices. To properly handle writes to the
register, export a pci_emul_cmd_changed function from pci_emul.c that
the pass through device model invokes for config writes that change
PCIR_COMMAND.

Reviewed by: rgrimes
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20531