Currently every caller of the brXXX APIs has to store the returned
errno value and then raise an error message. This results in
inconsistent error messages across drivers, additional burden on
the callers and makes the error reporting inaccurate since it is
hard to distinguish different scenarios from 1 errno value.

* src/util/bridge.c: Raise errors instead of returning errnos
* src/lxc/lxc_driver.c, src/network/bridge_driver.c,
  src/qemu/qemu_command.c, src/uml/uml_conf.c,
  src/uml/uml_driver.c: Remove error reporting code