iotests: 046: Prepare for image locking

The qemu-img info command is executed while VM is running, add -U option
to avoid the image locking error.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/046

@@ -192,7 +192,7 @@ echo "== Verify image content =="
 
 function verify_io()
 {
-    if ($QEMU_IMG info -f "$IMGFMT" "$TEST_IMG" | grep "compat: 0.10" > /dev/null); then
+    if ($QEMU_IMG info -U -f "$IMGFMT" "$TEST_IMG" | grep "compat: 0.10" > /dev/null); then
         # For v2 images, discarded clusters are read from the backing file
         # Keep the variable empty so that the backing file value can be used as
         # the default below