qapi/char: Make backend types properly conditional

Character backends are actually QOM types.  When a backend's
compile-time conditional QOM type is not compiled in, creation fails
with "'FOO' is not a valid char driver name".  Okay, except
introspecting chardev-add with query-qmp-schema doesn't work then: the
backend type is there even though the QOM type isn't.

A management application can work around this issue by using
qom-list-types instead.

Fix the issue anyway: add the conditionals to the QAPI schema.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240203080228.2766159-4-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qapi/char.json

@@ -472,8 +472,8 @@
 ##
 { 'enum': 'ChardevBackendKind',
   'data': [ 'file',
-            'serial',
-            'parallel',
+            { 'name': 'serial', 'if': 'HAVE_CHARDEV_SERIAL' },
+            { 'name': 'parallel', 'if': 'HAVE_CHARDEV_PARALLEL' },
             'pipe',
             'socket',
             'udp',
@@ -482,10 +482,10 @@
             'mux',
             'msmouse',
             'wctablet',
-            'braille',
+            { 'name': 'braille', 'if': 'CONFIG_BRLAPI' },
             'testdev',
             'stdio',
-            'console',
+            { 'name': 'console', 'if': 'CONFIG_WIN32' },
             { 'name': 'spicevmc', 'if': 'CONFIG_SPICE' },
             { 'name': 'spiceport', 'if': 'CONFIG_SPICE' },
             { 'name': 'qemu-vdagent', 'if': 'CONFIG_SPICE_PROTOCOL' },
@@ -614,8 +614,10 @@
   'base': { 'type': 'ChardevBackendKind' },
   'discriminator': 'type',
   'data': { 'file': 'ChardevFileWrapper',
-            'serial': 'ChardevHostdevWrapper',
-            'parallel': 'ChardevHostdevWrapper',
+            'serial': { 'type': 'ChardevHostdevWrapper',
+                        'if': 'HAVE_CHARDEV_SERIAL' },
+            'parallel': { 'type': 'ChardevHostdevWrapper',
+                          'if': 'HAVE_CHARDEV_PARALLEL' },
             'pipe': 'ChardevHostdevWrapper',
             'socket': 'ChardevSocketWrapper',
             'udp': 'ChardevUdpWrapper',
@@ -624,10 +626,12 @@
             'mux': 'ChardevMuxWrapper',
             'msmouse': 'ChardevCommonWrapper',
             'wctablet': 'ChardevCommonWrapper',
-            'braille': 'ChardevCommonWrapper',
+            'braille': { 'type': 'ChardevCommonWrapper',
+                         'if': 'CONFIG_BRLAPI' },
             'testdev': 'ChardevCommonWrapper',
             'stdio': 'ChardevStdioWrapper',
-            'console': 'ChardevCommonWrapper',
+            'console': { 'type': 'ChardevCommonWrapper',
+                         'if': 'CONFIG_WIN32' },
             'spicevmc': { 'type': 'ChardevSpiceChannelWrapper',
                           'if': 'CONFIG_SPICE' },
             'spiceport': { 'type': 'ChardevSpicePortWrapper',