vga-pci: add qext region to mmio

Add a qemu extented register range to the standard vga mmio bar.
Right nowe there are two registers:  One readonly register returning the
size of the region (so we can easily add more registers there if needed)
and one endian control register, so guests (especially ppc) can flip
the framebuffer endianness as they need it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

docs/specs/standard-vga.txt

@@ -70,3 +70,12 @@ Likewise applies to the pci variant only for obvious reasons.
 0500 - 0515 : bochs dispi interface registers, mapped flat
               without index/data ports.  Use (index << 1)
               as offset for (16bit) register access.
+
+0600 - 0607 : qemu extended registers.  qemu 2.2+ only.
+              The pci revision is 2 (or greater) when
+              these registers are present.  The registers
+              are 32bit.
+  0600      : qemu extended register region size, in bytes.
+  0604      : framebuffer endianness register.
+              - 0xbebebebe indicates big endian.
+              - 0x1e1e1e1e indicates little endian.