If a domain name is defined for a network, add the --expand-hosts
option to the dnsmasq commandline. This results in the domain being
added to any hostname that is defined in a dns <host> element and
contains no '.' characters (i.e. it is an "unqualified"
hostname). Since PTR records are automatically created for any name
defined in <host>, the result of a PTR request will change from the
unqualified name to the qualified name.

This also has the same effect on any hostnames that dnsmasq reads
from the host's /etc/hosts file.

(In the case of guest hostnames that were learned by dnsmasq via DHCP
requests, they were already getting the domain name added on, even
without --expand-hosts).