tp-libvirt: Fix iface-trans

Using `locate` utility to find a file could result into undesired
results and it also depends on whether the test system is indexed
properly. To find the `netcf-transaction` script, we use the autotest
`find_command()` to do the job. For system using SYSV init service
manager, we hard code the script location for a work around.

Also, alway cleanup enviroment in case a broken job influence
consecutive jobs.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #160 from Hao-Liu/fix_iface_trans

tp-libvirt: Fix iface-trans