ioemu: sdl blitting

ioemu: sdl blitting

Right now qemu takes care of converting pixels between the guest pixel
format and the sdl pixel format, after that qemu also memcpy the
converted pixels to the sdl buffer in video ram. This process can be
improved using the SDL blit capabilities: the patch I am attaching
creates an SDL Surface from the Cirrus VGA framebuffer and uses SDL
blitting functions to convert and write pixels to video ram. SDL
blitting functions are optimized and can even be hardware accelerated
on some platforms.

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