* docs/schemas/domain.rng: augment the video model with an optional
  acceleration element with optional accel2d and accel3d flags
* src/domain_conf.c src/domain_conf.h: exten the virDomainVideoDef
  structure with an optional accel field, virDomainVideoAccelDefParseXML
  and virDomainVideoAccelDefFormat functions to parse and serialize
  the structure.