To find out where the net type 'direct' needs to be handled I introduced
the 'enum virDomainNetType' in the virDomainNetDef structure and let the
compiler tell me where the case statement is missing. Then I added the
unhandled device statement to the UML driver.

* src/conf/domain_conf.h: change _virDomainNetDef type from int to
  virDomainNetType enum
* src/conf/domain_conf.c src/lxc/lxc_driver.c src/qemu/qemu_conf.c
  src/uml/uml_conf.c: make sure all enum cases are properly handled
  in switches