Pure code motion; no semantic change.

* src/conf/domain_conf.h (virDomainSeclabelType)
(virSecurityLabelDefPtr): Declare earlier.
* src/conf/domain_conf.c (virSecurityLabelDefClear)
(virSecurityLabelDefParseXML): Move earlier.
(virDomainDefParseXML): Move seclabel parsing earlier.