numa: Clean up query-memdev error handling

qmp_query_memdev() has two error paths:

* When object_get_objects_root() returns null.  It never does, so
  simply drop the useless error handling.

* When query_memdev() fails.  It leaks err then.  But any failure
  there is actually a programming error.  Switch it to &error_abort,
  and drop the useless error handling.

Messed up in commit 76b5d850 "qmp: add query-memdev".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>