Add #if defined(CONFIG_REPLICATION) in generated code, and adjust the
code accordingly.

Made conditional:

* xen-set-replication, query-xen-replication-status,
  xen-colo-do-checkpoint

  Before the patch, we first register the commands unconditionally in
  generated code (requires a stub), then conditionally unregister in
  qmp_unregister_commands_hack().

  Afterwards, we register only when CONFIG_REPLICATION.  The command
  fails exactly the same, with CommandNotFound.

  Improvement, because now query-qmp-schema is accurate, and we're one
  step closer to killing qmp_unregister_commands_hack().

* enum BlockdevDriver value "replication" in command blockdev-add

* BlockdevOptions variant @replication

and related structures.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-23-marcandre.lureau@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>