error, qerror: add ErrorClass argument to error functions

The new argument is added to functions qerror_report() and error_set().
It's stored in Error and QError. qerror_report_err() is also updated to
take care of it.

The QERR_ macros are changed to contain a place holder value for the
new argument, so that the value is used on all current calls to
qerror_report() and error_set() (and also to initialize qerror_table[]).

Next commit will update the QERR_ macros with a proper ErrorClass
value.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>