migration: Disable hotplug/unplug during migration

Until we have reviewed what can/can't be hotplugged during migration,
disable it.  We can enable it later for the things that we know that
work.  For instance, memory hotplug during postcopy doesn't work
currently.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>

--

- Fix typo.  Thanks Thomas.
- Delay migration check after we have checked that we can hotplug that
  device.
- more typos
