sdl: Reverse support for video mode setting

sdl: Reverse support for video mode setting

Currently, If the setting of video mode failed, qemu will exit. It
should go back to the previous setting if the new screen resolution
failed. This patch fixes LP#1216368, add support to revert to existing
surface for the failure of video mode setting.

Reported-by: Sascha Krissler <sascha@srlabs.de>
Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1378285636-7091-1-git-send-email-lilei@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>