gtk: Remember the last grabbed pointer position

It's pretty annoying that the pointer reappears at a random place once
after grabbing and ungrabbing the input.  Better to restore to the
original position where the pointer was grabbed.

Reference: https://bugzilla.novell.com/show_bug.cgi?id=849587
Tested-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>
Tested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
