ui: Print available display backends with '-display help'

We already print availabled devices with "-device help", or available
backends with "-netdev help" or "-chardev help". Let's provide a way
for the users to query the available display backends, too.
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-id: 20200108144702.29969-1-thuth@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

include/ui/console.h

@@ -442,6 +442,7 @@ void qemu_display_register(QemuDisplay *ui);
 bool qemu_display_find_default(DisplayOptions *opts);
 void qemu_display_early_init(DisplayOptions *opts);
 void qemu_display_init(DisplayState *ds, DisplayOptions *opts);
+void qemu_display_help(void);
 
 /* vnc.c */
 void vnc_display_init(const char *id, Error **errp);

qemu-options.hx

@@ -1669,7 +1669,8 @@ STEXI
 @item -display @var{type}
 @findex -display
 Select type of display to use. This option is a replacement for the
-old style -sdl/-curses/... options. Valid values for @var{type} are
+old style -sdl/-curses/... options. Use @code{-display help} to list
+the available display types. Valid values for @var{type} are
 @table @option
 @item sdl
 Display video output via SDL (usually in a separate graphics

ui/console.c

@@ -2333,6 +2333,21 @@ void qemu_display_init(DisplayState *ds, DisplayOptions *opts)
     dpys[opts->type]->init(ds, opts);
 }
 
+void qemu_display_help(void)
+{
+    int idx;
+
+    printf("Available display backend types:\n");
+    for (idx = DISPLAY_TYPE_NONE; idx < DISPLAY_TYPE__MAX; idx++) {
+        if (!dpys[idx]) {
+            ui_module_load_one(DisplayType_str(idx));
+        }
+        if (dpys[idx]) {
+            printf("%s\n",  DisplayType_str(dpys[idx]->type));
+        }
+    }
+}
+
 void qemu_chr_parse_vc(QemuOpts *opts, ChardevBackend *backend, Error **errp)
 {
     int val;

vl.c

@@ -1869,6 +1869,11 @@ static void parse_display(const char *p)
 {
     const char *opts;
 
+    if (is_help_option(p)) {
+        qemu_display_help();
+        exit(0);
+    }
+
     if (strstart(p, "sdl", &opts)) {
         /*
          * sdl DisplayType needs hand-crafted parser instead of