This patch makes libvirtd start the dnsmasq daemon with a
--dhcp-hostsfile option instead of --dhcp-host options for each
'//ip/dhcp/host' entries defined in network xml file.

the dnsmasq host file is stored into /var/lib/libvirt/network

* src/network/bridge_driver.c: define the directory for the hostfiles
  and save/delete them to be used by dnsmasq