Some configuration setups for guests are allowed, but strongly
discouraged and unsupportable in production systems. Introduce
a concept of 'tainting' to virDomainObjPtr to allow such setups
to be identified. Drivers can then log warnings at suitable
times

* src/conf/domain_conf.c, src/conf/domain_conf.h: Declare taint
  flags and add parsing/formatting of domain status XML