Ability to mount Virtio-P9 filesystems

Ability to mount Virtio-P9 filesystems. It would provide better peformance and reliability than NFS under KVM, bhyve and perhaps other hypervisors too.

There seems to be something Virtio-P9 -related already, but it is something specific to LXD. At the moment, just having a Virtio-9P device causes boot process to hang in bhyve.