Add virtio-9p (aka VirtFS) filesystem sharing to bhyve.

Add virtio-9p (aka VirtFS) filesystem sharing to bhyve.

VirtFS allows sharing an arbitrary directory tree between bhyve virtual
machine and the host. Current implementation has a fairly complete support
for 9P2000.L protocol, except for the extended attribute support. It has
been verified to work with the qemu-kvm hypervisor.

Reviewed by:	rgrimes, emaste, jhb, trasz
Approved by:	trasz (mentor)
MFC after:	1 month
Relnotes:	yes
Sponsored by:	Conclusive Engineering (development), vStack.com (funding)
Differential Revision:	https://reviews.freebsd.org/D10335