spice: drop incorrect vm_change_state_handler() opaque

The spice_server pointer is a global variable and
vm_change_state_handler() therefore does not use its opaque parameter.

The vm change state handler is added with a pointer to the spice_server
pointer.  This is useless and we probably would not want 2 levels of
pointers.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Uri Lublin <uril@redhat.com>