qapi: New QAPISourceInfo, replacing dict

qapi: New QAPISourceInfo, replacing dict

We track source locations with a dict of the form

    {'file': FNAME, 'line': LINENO, 'parent': PARENT}

where PARENT is None for the main file, and the include directive's
source location for included files.

This is serviceable enough, but the next commit will add information,
and that's going to come out cleaner if we turn this into a class.  So
do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-4-armbru@redhat.com>