qemu.tests.cfg: Add image format filter in qemu_iotests

Add image format filter for qemu_iotests. So we can filter the right
test cases when we run it.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #50 from ypu/qemu_iotests

qemu.tests.cfg: Add image format filter in qemu_iotests