iotests/common.rc: introduce _qcow2_dump_header helper

We'll use it in tests instead of explicit qcow2.py. Then we are going
to add some filtering in _qcow2_dump_header.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20211223160144.1097696-14-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

tests/qemu-iotests/common.rc

@@ -996,5 +996,15 @@ _require_one_device_of()
     _notrun "$* not available"
 }
 
+_qcow2_dump_header()
+{
+    img="$1"
+    if [ -z "$img" ]; then
+        img="$TEST_IMG"
+    fi
+
+    $PYTHON qcow2.py "$img" dump-header
+}
+
 # make sure this script returns success
 true