sdl: Do not make full screen mode resizable

This prevents continuous resizing events and improper screen setups when
going full screen.

CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
