MAINTAINERS: new maintainers for qapi-schema.json

MAINTAINERS: new maintainers for qapi-schema.json

I'm facing two problems lately wrt QMP patch review: increasingly
lack of bandwidth and lack of background in so many different areas
that are getting new QMP commands almost every week.

In order to help me mitigate this problem, I'm adding Eric and Markus
(besides me) as maintainers of the qapi-schema.json file.

Markus has been an old timer reviewer. Eric is being the most active
and prolific reviewer of QMP patches for some time now.

I believe Markus and Eric will keep doing their work as before, but
starting now I'll require the ACK of at least one of them before
appling a patch/series that touches the qapi-schema.json file.

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