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