Run radvd for virtual networks with IPv6 addresses

Run radvd for virtual networks with IPv6 addresses

Running an instance of the router advertisement daemon (radvd) allows
guests using the virtual network to automatically acquire an IPv6
address and default route. Note that acquiring an address only works
for networks with a prefix length of exactly 64 - radvd is still run
in other circumstances, and still advertises routes, but autoconf will
not work because it requires exactly 64 bits of address info from the
network prefix.

This patch avoids a race condition with the pidfile by manually
daemonizing radvd rather than allowing it to daemonize itself, then
creating our own pidfile (in addition to radvd's own file, which is
unnecessary, but there is no way to tell radvd to not create it). This
is accomplished by exec'ing it with "--debug 1" in the commandline,
and using virCommand's features to fork, create a pidfile, and detach
from the newly forked process.