qemu.tests: Set restart_network to True when check guest network

As the link up and down in set_link may cause ip change in some enviroment.
We need add restart_network when we try to login guest after the operations.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #85 from ypu/set_link

qemu.tests: Set restart_network to True when check guest network