bhyve: Split out a lower-level helper for VirtIO interrupts.

This allows device models to assert VirtIO interrupts for reasons
other than publishing changes to a VirtIO ring such as configuration
changes.

Reviewed by: grehan, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30505

(cherry picked from commit c06676bee3d228ac18c5ed3604304e932eb84c1e)