sdl: Fix memory leakage

Valgrind was so kind to remark that no one bothers to release keycodes
after use and that something is fishy about cleaning up the requested
keyboard descriptor. With this patch applied, we no longer leak about
12k during startup.

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