iotests/image-fleecing: declare requirement of copy-before-write

Now test fails if copy-before-write is not white-listed.
Let's skip test instead.

Fixes: c0605985
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210920115538.264372-6-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/tests/image-fleecing

@@ -28,6 +28,7 @@ from iotests import log, qemu_img, qemu_io, qemu_io_silent
 iotests.script_initialize(
     supported_fmts=['qcow2', 'qcow', 'qed', 'vmdk', 'vhdx', 'raw'],
     supported_platforms=['linux'],
+    required_fmts=['copy-before-write'],
 )
 
 patterns = [('0x5d', '0',         '64k'),