qom: provide root container for internal objs

qom: provide root container for internal objs

We have object_get_objects_root() to keep user created objects, however
no place for objects that will be used internally.  Create such a
container for internal objects.

CC: Andreas Färber <afaerber@suse.de>
CC: Markus Armbruster <armbru@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170928025958.1420-2-peterx@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>