bhyve: add bus, slot and func to device name

Each device needs a unique identifier to store and restore snapshots
properly. Adding the pci bsf information to the device name creates a
unique identifier as a bsf can't be occupied twice.

Reviewed by: corvink
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40107

(cherry picked from commit 14c80457b0f6a5eb6ff9a1db573462410c64a454)