fstests: Add virtio-fs shared file system support

fstests: Add virtio-fs shared file system support

This patch adds support for virtio-fs shared file system that lets
virtual machines access a directory tree on the host.

To run xfstests on it, first, start virtiofsd daemon in host:
 ./virtiofsd -o vhost_user_socket=/tmp/vhostqemu0 -o source=$DIR0 -o cache=always
 ./virtiofsd -o vhost_user_socket=/tmp/vhostqemu1 -o source=$DIR1 -o cache=always

second, launch QEMU with:
 -chardev socket,id=char0,path=/tmp/vhostqemu0
 -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs0
 -chardev socket,id=char1,path=/tmp/vhostqemu1
 -device vhost-user-fs-pci,queue-size=1024,chardev=char1,tag=myfs1
 -m 8G
 -object memory-backend-file,id=mem,size=8G,mem-path=/dev/shm,share=on
 -numa node,memdev=mem

then, inside the VM run xfstests with:
 export TEST_DEV=myfs0
 export TEST_DIR=$TESTDIR
 export SCRATCH_DEV=myfs1
 export SCRATCH_MNT=$SCRATCHMNT
 export MOUNT_OPTIONS=""
 export TEST_FS_MOUNT_OPTS=""

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Misono Tomohiro <misono.tomohiro@fujitsu.com>
Signed-off-by: QI Fuli <qi.fuli@fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>