qapi: Fix error message format regression

qapi: Fix error message format regression

Commit 52a474180ae3 changed reporting of errors connected to a source
location without mentioning it in the commit message.  For instance,

    $ python scripts/qapi-gen.py tests/qapi-schema/unknown-escape.json
    tests/qapi-schema/unknown-escape.json:3:21: unknown escape \x

became

    scripts/qapi-gen.py: tests/qapi-schema/unknown-escape.json:3:21: unknown escape \x

This is not how compilers report such errors, and Emacs doesn't
recognize the format.  Revert this change.

Fixes: 52a474180ae3 (qapi-gen: Separate arg-parsing from generation)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>