tests: vmstate static checker: add substructure for usb-kbd for hid section

This shows how the script deals with substructures added to vmstate
descriptions that don't change the on-wire format.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
