nic_hotplug: needs booting guest without nic before hot-plug

According to the test case, needs booting guest
without nic before nic hot-plug.

Signed-off-by: Cong Li <coli@redhat.com>
Merge pull request #105 from CongSmile/nic_hotplug

nic_hotplug: needs booting guest without nic before nic hot-plug