conf: Don't allow multiple seclabels for same model

conf: Don't allow multiple seclabels for same model

https://bugzilla.redhat.com/show_bug.cgi?id=1066894

With current code it's possible to have for instance:

virsh dumpxml mydomain | grep seclabel
  <seclabel type='dynamic' model='selinux' relabel='yes'/>
  <seclabel type='dynamic' model='selinux' relabel='yes'/>
  <seclabel type='dynamic' model='selinux' relabel='yes'/>
  <seclabel type='dynamic' model='selinux' relabel='yes'/>
  <seclabel type='dynamic' model='selinux' relabel='yes'/>

what doesn't make any sense. We should reject the XML in the config
parsing phase.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>