xfs/078: instead file image by mkfs on loopback device

xfs/078: instead file image by mkfs on loopback device

If test on 4k sector size device, xfs/078 will fail when it try to
make a filesystem image with block size less than 4096. But if we
attach the file image to a loop device, it can accept 512 block
size. So this patch attach a loop device before do mkfs.xfs.

[eguan: replace bare mount/umount with [U]MOUNT_PROG]

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>