qapi: Improve reporting of member name clashes

qapi: Improve reporting of member name clashes

We report name clashes like this:

    struct-base-clash.json: In struct 'Sub':
    struct-base-clash.json:5: 'name' (member of Sub) collides with 'name' (member of Base)

The "(member of Sub)" is redundant with "In struct 'Sub'".  Comes from
QAPISchemaMember.describe().  Pass info to it, so it can detect the
redundancy and avoid it.  Result:

    struct-base-clash.json: In struct 'Sub':
    struct-base-clash.json:5: member 'name' collides with member 'name' of type 'Base'

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-8-armbru@redhat.com>