network: detect conflicting route even if it is the final entry

network: detect conflicting route even if it is the final entry

This is a folloup to commit 5f719596, which checks for a route
conflicting with the standard libvirt default network subnet
(192.168.122.0/24). It turns out that $() strips the trailing newline
from the output of "ip route show", so there would be no match if the
route we were looking for was the final line of output. This can be
solved by adding ${nl} to the end of the output (just as we were
already adding it at the beginning of the output).