spice-display: fix segfault in qemu_spice_create_update

Although it is pretty unusual the stride for the guest image and the
mirror image maintained by spice-display can be different.  So use
separate variables for them.

https://bugzilla.redhat.com/show_bug.cgi?id=1163047

Cc: qemu-stable@nongnu.org
Reported-by: perrier vincent <clownix@clownix.net>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
