network: restart dnsmasq after adding/removing txt and srv records
    
    Although dns host records are stored in a separate configuration file
    that is reread by dnsmasq when it receives a SIGHUP, the txt and srv
    records are directly in the dnsmasq .conf file which can't be reread
    after initial dnsmasq startup. This means that if an srv or txt record
    is modified in a network config, libvirt needs to restart the dnsmasq
    process rather than just sending a SIGHUP.
    
    This was pointed out in a question in
    https://bugzilla.redhat.com/show_bug.cgi?id=988718 , but no separate
    BZ was filed.