cirrus: handle wraparound in cirrus_invalidate_region

cirrus: handle wraparound in cirrus_invalidate_region

Code simply asserts that there is no wraparound instead of handling
it properly.  The assert() can be triggered by the guest (must be
privilidged inside the guest though).  Fix it.

Buglink: https://bugs.launchpad.net/qemu/+bug/1880189
Cc: Li Qiang <liq3ea@163.com>
Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Li Qiang <liq3ea@163.com>
Message-id: 20200901140944.24101-1-kraxel@redhat.com