conf: Move validation of virDomainGraphicsListenDef out of parser
In an effort to separate the validation steps from the Parse stage,
a few validation checks of virDomainGraphicsListenDef have been moved from
virDomainGraphicsListenDefParseXML() in domain_conf.c to
virDomainGraphicsDefListensValidate() in domain_validate.c
Signed-off-by: K Shiva <shiva_kr@riseup.net> Signed-off-by: Michal Privoznik <mprivozn@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>