shared.cfg: Check SSDT for both S3/S4 tests

The SSDT shows whether the bios set the suspend status
correctly, so it's another test point for both S3/S4
tests.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #639 from zhouqt/suspend

shared.cfg: Check SSDT for both S3/S4 tests
virttest.utils_net : modify the method of get_linux_ifname

This patch modify the method of get_linux_ifname
we can get the all the nic interface in the guest or
only get the nic interface with mac address we assigned.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>