Implementations of following functions:
  virDomainVcpupinIsDuplicate
  virDomainVcpupinFindByVcpu
  virDomainVcpupinAdd

Update "virDomainDefParseXML" to parse, and "virDomainDefFormatXML"
to build cputune xml, also implementations of new internal helper
functions.

v1 - v2:
  * Resolve potential crash bug of "virDomainVcpupinAdd"