tp-libvirt: Fix macvtap

* Replace the hardcoded assumption of interface name and config file path
  to `ENTER.YOUR...`, and check all there parameters before test.

* If the target interface is not exist on the host. we SKIP the test.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #170 from Hao-Liu/fix_macvtap

tp-libvirt: Fix macvtap