qcow2: fix leak of Qcow2DiscardRegion in update_refcount_discard

When the Qcow2DiscardRegion is adjacent to another one referenced by "d",
free this Qcow2DiscardRegion metadata referenced by "p" after
it was removed from s->discards queue.

Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-refcount.c

@@ -524,6 +524,7 @@ found:
         QTAILQ_REMOVE(&s->discards, p, next);
         d->offset = MIN(d->offset, p->offset);
         d->bytes += p->bytes;
+        g_free(p);
     }
 }