qxl: properly handle upright and non-shared surfaces

Although qxl creates a shared displaysurface when the qxl surface is
upright and doesn't need to be flipped there is no guarantee that the
surface doesn't become unshared for some reason.  Rename qxl_flip to
qxl_blit and fix it to handle both flip and non-flip cases.

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