Fix XVM's classic networking problem

Fix XVM's classic networking problem

vif-invirtroute: Kill arpspoof with SIGKILL when we're done.  The
version in this distro (which is the latest beta from the
unmaintained upstream) tries to "clean up" when you normally
terminate it.  This involves trying to perform an ARP lookup
without interfering to see what the MAC should've been and then
sending replies for that.  This appears to cause a race depending
on whether the entry for the machine's old domain (if it's being
rebooted) is listed as <incomplete> in the ARP table or is totally
flushed.  If you lose the race, it sends further gratuitous ARPs
telling the router that your MAC is all-zeros because of a bug
where it fails to check for an incomplete ARP entry.  This is
believed to cause the bug that has plagued XVM for years where
you sometimes lose off-network connectivity on reboot.