sdl2: fix hotkey keyup

After some hotkey was pressed sdl2 doesn't forward the first modifier
keyup event to the guest, resulting in stuck modifier keys.

Fix the logic in handle_keyup().  Also gui_key_modifier_pressed doesn't
need to be a global variable.

Reported-by: Howard Spoelstra <hsp.cat7@gmail.com>
Tested-by: Howard Spoelstra <hsp.cat7@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180220150444.784-1-kraxel@redhat.com