Fix building domain def in securityselinuxtest

Fix building domain def in securityselinuxtest

The virDomainDef created by testBuildDomainDef in securityselinuxtest
adds a seclabel but does not increment nseclabels. Also, it should
populate seclabel->model with 'selinux'.

While at it, use the secdef itself to populate values instead of
the indirection through def->seclabels[0].