tp-libvirt: Skip test if tgtadm is not found

When the optional binary `tgtadm` is not available on the host, we don't
expect the test case fail and should skip it instead.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #158 from Hao-Liu/fix_attach_detach_disk

tp-libvirt: Skip test if tgtadm is not found