bhyve: Keep passthrough PCI power state virtual

[kbowling]

Summary

The passthrough Command register is emulated, but PMCSR writes were
sent directly to the physical function.  A guest D3hot-to-D0 transition
can perform an internal reset and clear physical Command while its
emulated copy remains enabled.

Cache the Power Management capability and keep the physical D-state
host-owned.  Emulate the guest D-state and advertise No_Soft_Reset so
the guest is not promised a function reset by a virtual power cycle.
Restore the assignment-time virtual state after a managed FLR.

MFC after:      2 weeks

[markj]

usr.sbin/bhyve/pci_passthru.c

1319

Is this an unrelated cleanup?

1333

It'd be good to introduce a helper function which factors out this expression, with a comment explaining it, as it appears quite a few times and isn't easy to read.

[kbowling]

usr.sbin/bhyve/pci_passthru.c

1319

it's related to the immediate predecessor, this 0xffff check was for device removal.