softmmu/vl.c: Check for the availability of the VGA device before using it

In case the user disabled the default VGA device in the binary (e.g.
with the "--without-default-devices" configure switch), we should
not try to use it by default if QEMU is running with the default
devices, otherwise it aborts when trying to use it. Simply emit a
warning instead.

Message-Id: <20230512124033.502654-3-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

softmmu/vl.c

@@ -957,7 +957,18 @@ static const char *
 get_default_vga_model(const MachineClass *machine_class)
 {
     if (machine_class->default_display) {
-        return machine_class->default_display;
+        for (int t = 0; t < VGA_TYPE_MAX; t++) {
+            const VGAInterfaceInfo *ti = &vga_interfaces[t];
+
+            if (ti->opt_name && vga_interface_available(t) &&
+                g_str_equal(ti->opt_name, machine_class->default_display)) {
+                return machine_class->default_display;
+            }
+        }
+
+        warn_report_once("Default display '%s' is not available in this binary",
+                         machine_class->default_display);
+        return NULL;
     } else if (vga_interface_available(VGA_CIRRUS)) {
         return "cirrus";
     } else if (vga_interface_available(VGA_STD)) {