qapi: add conditions to SPICE type/commands/events on the schema

Add #if defined(CONFIG_SPICE) in generated code, and adjust the
qmp/hmp code accordingly.

query-qmp-schema no longer reports the command/events etc as
available when disabled at compile time.

Commands made conditional:

* query-spice

  Before the patch, the command for !CONFIG_SPICE is unregistered. It
  will fail with the same error.

Events made conditional:

* SPICE_CONNECTED, SPICE_INITIALIZED, SPICE_DISCONNECTED,
  SPICE_MIGRATE_COMPLETED

Add TODO for conditional SPICE chardevs, delayed until the supports
for conditional members lands.

No HMP change, the code was already conditional.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180703155648.11933-15-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>