bhyve: missing mutex initializations

Explicitly initialize the mutex that a PCI virtio module passes back to
virtio.

It so happens that these mutexes were being initialized regardless, no
functional change intended.

Reviewed by: chuck, jhb
Differential Revision: https://reviews.freebsd.org/D34372

(cherry picked from commit f6f357efb1067b79678d8f348333ffdfec66ad20)