As in the case of nbd_export_new(), bdrv_invalidate_cache() can be
called when migration is still in progress. In this case we are not
ready to tighten the shared permissions fenced by blk->disable_perm.

Defer to a VM state change handler.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20170815130740.31229-4-famz@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>