qemu.test.set_link : Make set_link test support guest nic operstate check

This patch make set_link test support guest nic operstate check, both
guest nic boot with/without 'status' option.

This patch also add guest nic operstate and guest network connecting
check after reboot.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #1034 from humanux/set_link_v1

qemu.test.set_link : Make set_link test support guest nic operstate check.