tests.nicdriver_unload: Make the guest nic also named eth0

When run nicdriver_unload test, will raise error like:
013-04-16 21:35:55: modprobe virtio_net
2013-04-16 21:35:56: [root@unused ~]#
2013-04-16 21:35:56: echo $?
2013-04-16 21:35:56: 0
2013-04-16 21:35:56: [root@unused ~]#
2013-04-16 21:35:56: ifconfig eth4 up
2013-04-16 21:35:56: eth4: unknown interface: No such device
2013-04-16 21:35:56: [root@unused ~]#
2013-04-16 21:35:56: echo $?
2013-04-16 21:35:56: 255
2013-04-16 21:35:56: [root@unused ~]#
2013-04-16 21:35:56: udev: renamed network interface eth0 to eth4

the root cause of this cause is after the nic driver loaded,
the nic name not changed immediately by udev rules, because
the guest is under heavy working pressure.

This patch modify the guest udev net rules before test,
make guest nic always nemed eth0.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>