vl: Drain before (block) job cancel when quitting

vl: Drain before (block) job cancel when quitting

If the main loop cancels all block jobs while the block layer is not
drained, this cancelling may not happen instantaneously.  We can start a
drained section before vm_shutdown(), which entails another
bdrv_drain_all(); this nested bdrv_drain_all() will thus be a no-op,
basically.

We do not have to end the drained section, because we actually do not
want any requests to happen from this point on.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>