MAINTAINERS: New maintainer for QMP and QAPI

Markus is taking over maintership of QMP and the QAPI from
me. Markus has always been a great reviewer and contributor
to those subsystems. In the last few months he's also doing
pull requests that are a lot more relevant than the ones I
was able to do. So, this is a natural move.

I'm still the maintainer of HMP and QObjects, but I'm
looking for someone to take over those too.

PS: This commit also fixes the file listing for the QMP
    entry.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>