qemu: disable sdl and documentation

qemu: disable sdl and documentation

A newer version of qemu does not build anymore with our version of sdl. I
tried around a little bit and as I have not got a clue why we are using
sdl (spice and remote access still works)  I think we should disable it.

I disabled the generation of the documentation as well but this switch
does not seem to have any effect.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>