qemu-replies-tool: Add validation of known fields in 'query-qmp-schema'

qemu-replies-tool: Add validation of known fields in 'query-qmp-schema'

If the schema itself is extended in qemu we need to have a notification
to add appropriate handling to ensure that we have full coverage of all
fields.

Add validation that only fields that libvirt currently knows about are
present in the schema.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>