tp-libvirt: Fix libvirt_pci_passthrough

* Fixed a bug when no storage device name matches target name, it can't
  properly raise an exception

* Alway SKIP the test when the target node devices are not found, else
  the tests will FAIL if the default `ENTER.YOUR...` are not changed.

* When testing storage passthrough, alway check whether its parent node
  device is a PCI device. It might be `scsi` as well, and will get an
  unexpected behaviour.

Signed-off-by: Hao Liu <hliu@redhat.com>