vga: add mmio bar to standard vga

This patch adds a mmio bar to the qemu standard vga which allows to
access the standard vga registers and bochs dispi interface registers
via mmio.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/pc_piix.c

@@ -379,6 +379,10 @@ static QEMUMachine pc_machine_v1_3 = {
             .driver   = "qxl-vga",\
             .property = "revision",\
             .value    = stringify(3),\
+        },{\
+            .driver   = "VGA",\
+            .property = "mmio",\
+            .value    = "off",\
         }
 
 static QEMUMachine pc_machine_v1_2 = {