ui/console: add vc encoding=utf8/cp437 option

Expose a new "encoding" QemuOpt option.

Add the corresponding QAPI type and properties.

This is going to be wired in the following commits.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

chardev/char.c

@@ -650,6 +650,13 @@ ChardevBackend *qemu_chr_parse_opts(QemuOpts *opts, Error **errp)
             return NULL;
         }
     }
+    if (!cc->supports_encoding_opts) {
+        if (qemu_opt_get(opts, "encoding")) {
+            error_setg(errp, "chardev '%s' does not support encoding option",
+                       qemu_opts_id(opts));
+            return NULL;
+        }
+    }
 
     backend = g_new0(ChardevBackend, 1);
     backend->type = CHARDEV_BACKEND_KIND_NULL;
@@ -972,6 +979,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "rows",
             .type = QEMU_OPT_NUMBER,
+        },{
+            .name = "encoding",
+            .type = QEMU_OPT_STRING,
         },{
             .name = "mux",
             .type = QEMU_OPT_BOOL,