vl: Make display_type a local variable

Now display_type is only used inside main(), and don't need to be a
global variable.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

include/sysemu/sysemu.h

@@ -155,7 +155,6 @@ extern int vga_interface_type;
 extern int graphic_width;
 extern int graphic_height;
 extern int graphic_depth;
-extern DisplayType display_type;
 extern int display_opengl;
 extern const char *keyboard_layout;
 extern int win2k_install_hack;