vga: fix -nodefaults -device VGA

Flag -nodefaults should also imply no VGA. This was broken in
a369da5f31.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

vl.c

@@ -3006,6 +3006,7 @@ int main(int argc, char **argv, char **envp)
                 default_floppy = 0;
                 default_cdrom = 0;
                 default_sdcard = 0;
+                vga_model = "none";
                 break;
             case QEMU_OPTION_xen_domid:
                 if (!(xen_available())) {