call bdrv_drain_all() even if the vm is stopped

There are still I/O operations when the vm is stopped. For example,
stop the vm, and do block migration. In this case, we don't drain all
I/O operation, and may meet the following problem:

qemu-system-x86_64: migration/block.c:731: block_save_complete: Assertion `block_mig_state.submitted == 0' failed.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>
Message-Id: <564EE92E.4070701@cn.fujitsu.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -1415,6 +1415,8 @@ int vm_stop_force_state(RunState state)
         return vm_stop(state);
     } else {
         runstate_set(state);
+
+        bdrv_drain_all();
         /* Make sure to return an error if the flush in a previous vm_stop()
          * failed. */
         return bdrv_flush_all();