ioemu: Share framebuffer between VGA device model and VNC server.

ioemu: Share framebuffer between VGA device model and VNC server.

Now that the qemu WMVi patch is applied we can take full advantage of
it sharing the video buffer between the vga driver and the qemu vnc
server. This saves a lot of memcpy. It's worth mentioning again that
when the guest colour depth is 24 bit we cannot share the buffer
because 24 bpp is not supported by the vnc protocol, so we still have
to do the translation 24 bpp -> 32 bpp.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>