qxl: remove flipped

Tested on linux and windows guests. For negative stride, qxl_flip copies
directly to vga->ds->surface->data, for positive it's reallocated to
share qxl->guest_primary.data

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
