block: Drain requests in bdrv_close

If an AIO request is in flight that refers to a BlockDriverState that
has been closed and possibly even freed, more or less anything could
happen. I have seen segfaults, -EBADF return values and qcow2 sometimes
actually catches the situation in bdrv_close() and abort()s.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

block.c

@@ -816,6 +816,8 @@ void bdrv_close(BlockDriverState *bs)
         if (bs->job) {
             block_job_cancel_sync(bs->job);
         }
+        bdrv_drain_all();
+
         if (bs == bs_snapshots) {
             bs_snapshots = NULL;
         }