qcow2: insert assert into qcow2_get_specific_info()

s->qcow_version is always set to 2 or 3. Let's assert if this is wrong.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
CC: Roman Kagan <rkagan@virtuozzo.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -2803,6 +2803,10 @@ static ImageInfoSpecific *qcow2_get_specific_info(BlockDriverState *bs)
             .has_corrupt        = true,
             .refcount_bits      = s->refcount_bits,
         };
+    } else {
+        /* if this assertion fails, this probably means a new version was
+         * added without having it covered here */
+        assert(false);
     }
 
     return spec_info;