sdl: Dynamically grab input in absolute mouse mode

Not grabbing the input means that special keys like ALT+TAB are still
handled by the host. Improve the usability by grabbing input once the
mouse is inside the guest screen, provided the SDL window has the input
focus. Release it again when the mouse is moved to any border. Also grab
the input when we gain the input focus and the mouse is within the
screen limits.

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