block: for HMP commit() operations on 'all', skip non-COW drives

During a commit of 'all' using the HMP non-live commit, the operation
is aborted and returns error on the first error enountered.  When
non-COW drives are in use (e.g. ejected floppy, cdrom, or drives without
a backing parent), that means a commit all will return an error of either
-ENOMEDIUM or -ENOTSUP.  This is not desirable, so for the 'all' commit
case, only attempt the commit if both bs->drv and bs->backing_hd are
present.

More succinctly: 'commit all' now means a commit on all COW drives.

This means an individual commit to a specific non-COW drive will still
return the appropriate error (-ENOMEDIUM if eject / not present, -ENOTSUP
if no backing file).

Reported-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -1640,11 +1640,13 @@ int bdrv_commit_all(void)
     BlockDriverState *bs;
 
     QTAILQ_FOREACH(bs, &bdrv_states, list) {
+        if (bs->drv && bs->backing_hd) {
             int ret = bdrv_commit(bs);
             if (ret < 0) {
                 return ret;
             }
         }
+    }
     return 0;
 }