virttest.utils_test: get_linux_ifname - Make sys_method more robust

Sometimes, /sys/class/net may contain files (such as bonding_masters,
if the bonding module is loaded). So, let's handle the ShellCmdErrors
that might arise when trying to list the mac address of the interfaces.

Also, if no suitable interfaces were found, just return None.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.utils_test: get_linux_ifname - Introduce ip link method

Try to use ip link in case ifconfig didn't work. Looking
on /sys would be the last resource.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
libvirt tests.cfg: Remove OS definition

Since this prevents running things with the test runner
with other guests.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>