73-special-net-names.rules: Refine ibmveth naming

Stop assuming that the bus number always starts with '3', e. g. QEMU defaults
to '7'. Instead, now always just cut off the bus number prefix.  This assumes
that we won't get *two* virtual buses with conflicting device numbers, but
having multiple buses has never been observed.

If a device has number 'x000000', name it "ibmveth0" instead of just "ibmveth".

Add missing ' to terminate the sh -c '' command.

Thanks to Adam Conrad!