Unless we are running in full-screen mode, QEMU's SDL window should not
disable the host's screensaver. The user can still change this behaviour
by setting the environment variable SDL_VIDEO_ALLOW_SCREENSAVER as
desired.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>