qxl: fix guest cursor tracking

(1) If the guest cursor command is empty, don't reload it after migration.
(2) Cleaning the guest cursor when it is released by
    the spice server. In addition, explicitly reset the
    cursor in spice upon destroying the primary surface
    (was done by spice-server implicitly). This will prevent
    access to pci memory that was released.

RHBZ: 744518

Signed-off-by: Yonit Halperin <yhalperi@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
