sync DisplayAllocator interface

sync DisplayAllocator interface

this simple patch sync's the DisplayAllocator interface with upstream
qemu. The only non trivial change, is the new ability for sdl to support
the DisplayAllocator interface, hence improving performances (avoid a
memcpy) when the guest is in text mode or uses a color depth of 8bpp or
24bpp (no DisplayState surface sharing allowed).

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