Add virtio bus 'poll' method allowing us to inform backend we are
going to poll virtqueue.

Use on BERI soft-core to invalidate cpu caches.

Reviewed by:	bryanv
Sponsored by:	DARPA, AFRL


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275728 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f