fstests: Add 9p network filesystem support

fstests: Add 9p network filesystem support

This commit adds support for the 9p network file system, which is mainly
used by QEMU for sharing a file system from the host to the guest VM.

To run xfstests on it, launch QEMU with e.g.:

-virtfs local,path=$TMPDIR/p9-test,security_model=mapped-xattr,mount_tag=p9-test
-virtfs local,path=$TMPDIR/p9-scratch,security_model=mapped-xattr,mount_tag=p9-scratch

and inside the VM run xfstests with:

export TEST_DEV=p9-test
export SCRATCH_DEV=p9-scratch
export PLAN9_MOUNT_OPTIONS="-o trans=virtio,version=9p2000.L,cache=loose,posixacl"

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>