Convert dhcpStartDhcpDaemon from virRun to virCommand

Convert dhcpStartDhcpDaemon from virRun to virCommand

This is pretty straightforward - even though dnsmasq gets daemonized
and uses a pid file, those things are both handled by the dnsmasq
binary itself. And libvirt doesn't need any of the output of the
dnsmasq command either, so we just setup the args and call
virRun(). Mainly it was just a (mostly) mechanical job of replacing
the APPEND_ARG() macro (and some other *printfs()) with
virCommandAddArg*().