qemu.cfg.qemu_img: Change the way used to limit image_backend variant

"(image_backend=gluster):" is not supported. It is parsed as (image_backend = gluster):

So add "no (image_backend=gluster)" under to_qed variant

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1310 from FengYang/qemu_img2

qemu.cfg.qemu_img: Change the way used to limit image_backend variant