cleanup useless return sentence

This patch cleans up return sentences in the end of void functions.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

blockdev.c

@@ -830,7 +830,6 @@ exit:
     QSIMPLEQ_FOREACH_SAFE(states, &snap_bdrv_states, entry, next) {
         g_free(states);
     }
-    return;
 }