Re: [libvirt] [PATCHv2 15/21] conf: Add separate defaults addition and validation for XML parsing

This patch adds instrumentation that will ultimately allow to split out filling of defaults and input validation from the XML parser to separate functions. With this patch, after the XML is parsed, a callback to the driver is issued requesing to fill and validate driver specific details of the configuration. This allows to use sensible defaults and checks on a per driver basis at the time the XML is parsed. Three callback pointers are exposed in the new virDomainXMLConf object: * virDeviceDefAdjustCallback - called for a single device parsed and for every single device in a domain config. A virDomainDeviceDefPtr is passed along with the domain definition and virCaps. * virDomainDefAdjustCallback - called if a domain definition is parsed device specific callbacks were called. A virDomainDefPtr is passed along with virCaps. There are two callbacks of this kind, one called before the devices are parsed and one after. Errors may be reported in those callbacks resulting in a XML parsing failure. Additionally internal filler and checker functions are added that are meant to be used forseparating the validation and defaults assignment code from the parser function.