nic_hotplug: should boot guest without nic before hot-plug

1. nic_hotplug.cfg:
Have already updated the 'additional' and 'migration' part of nic_hotplug.cfg.

2. multi_nics_hotplug.cfg:
Update the single nic part.

Signed-off-by: Cong Li <coli@redhat.com>
Merge pull request #132 from CongSmile/nic_hotplug.vhost

nic_hotplug.vhost: should boot guest without nic before hot-plug