qcow: initialize coroutine mutex

commit 52b8eb60132b27ad53476490e9d7579003390cfa added a mutex,
but never initialized it.  This caused a segfault.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow.c

@@ -159,6 +159,8 @@ static int qcow_open(BlockDriverState *bs, int flags)
             goto fail;
         bs->backing_file[len] = '\0';
     }
+
+    qemu_co_mutex_init(&s->lock);
     return 0;
 
  fail: