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

[jhb]

Summary

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.

Test Plan

• Nick Principe tested this and it fixed his issue with Windows 10 and a pass through NIC

[rgrimes]

Looks like diff has gotten really confused by something making this very hard to see what it is that really changed since we are seeing function
pci_emul_cmdsts_write
compared to a new function
pci_emul_cmd_changed and are not actually seeing what changed in
pci_emul_cmdsts_write

Anyway to regenerate this in a way that makes it reviewable? never mind, looking at the raw diff it is clear as a bell, what Phabricator is doing is a mess.