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>
diff --git a/MAINTAINERS b/MAINTAINERS
index 96eaa1e..bf6742d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1264,6 +1264,7 @@
 M: Eduardo Habkost <ehabkost@redhat.com>
 M: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
 S: Supported
+F: hw/core/machine-qmp-cmds.c
 F: hw/core/machine.c
 F: hw/core/null-machine.c
 F: hw/core/numa.c