close all the block drivers before the qemu process exits

This patch calls the close handler of the block driver before the qemu
process exits.

This is necessary because the sheepdog block driver releases the lock
of VM images in the close handler.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

vl.c

@@ -1991,6 +1991,7 @@ static void main_loop(void)
             vm_stop(r);
         }
     }
+    bdrv_close_all();
     pause_all_vcpus();
 }