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

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

Add #if defined(CONFIG_VNC) 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.

Commands made conditional:

* query-vnc, query-vnc-servers, change-vnc-password

  Before the patch, the commands for !CONFIG_VNC are stubs that fail
  like this:

    {"error": {"class": "GenericError",
               "desc": "The feature 'vnc' is not enabled"}}

  Afterwards, they fail like this:

    {"error": {"class": "CommandNotFound",
               "desc": "The command FOO has not been found"}}

  I call that an improvement, because it lets clients distinguish
  between command unavailable (class CommandNotFound) and command failed
  (class GenericError).

Events made conditional:

* VNC_CONNECTED, VNC_INITIALIZED, VNC_DISCONNECTED

HMP change:

* info vnc

  Will return "unknown command: 'info vnc'" when VNC is compiled
  out (same as error for spice when --disable-spice)

Occurrences of VNC (case insensitive) in the schema that aren't
covered by this change:

* add_client

  Command has other uses, including "socket bases character devices".
  These are unconditional as far as I can tell.

* set_password, expire_password

  In theory, these commands could be used for managing any service's
  password.  In practice, they're used for VNC and SPICE services.
  They're documented for "remote display session" / "remote display
  server".

  The service is selected by argument @protocol.  The code special-cases
  protocol-specific argument checking, then calls a protocol-specific
  function to do the work.  If it fails, the command fails with "Could
  not set password".  It does when the service isn't compiled in (it's a
  stub then).

  We could make these commands conditional on the conjunction of all
  services [currently: defined(CONFIG_VNC) || defined(CONFIG_SPICE)],
  but I doubt it's worthwhile.

* change

  Command has other uses, namely changing media.
  This patch inlines a stub; no functional change.

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: <20180703155648.11933-14-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>