hw/core: Collect QMP command handlers in hw/core/

The handlers for qapi/machine.json's QMP commands are spread over
cpus.c, hw/core/numa.c, monitor/misc.c, monitor/qmp-cmds.c, and vl.c.
Move them all to new hw/core/machine-qmp-cmds.c, where they are
covered by MAINTAINERS section "Machine core", just like
qapi/machine.json.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190619201050.19040-11-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>