virtfs root filesystem support

virtfs root filesystem support

Qemu/KVM provides virtfs, a paravirtualised filesystem that is
implemented by running the Plan 9 folder sharing protocol over
virtio.

Make booting with root=virtfs:foobar use the virtfs filesystem
with mount tag 'foobar' as root filesystem, to allow booting
virtual machines off virtfs.

Note that this only handles 9p over virtio (i.e. virtfs), and
doesn't attempt to handle mounting 9p filesystems over TCP/IP,
for example.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>