bhyve: add .pe_snapshot method for PCI 'hostbridge'

There is no error when dump doesn't have it, but to be more
consistent this PCI devices should be saved as well.

Reviewed by: corvink, rew
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40108

(cherry picked from commit 6f7d2cf8bbb5ce6c4dbe0153d54cdc474b5266f4)