hw/block/xen: use proper format string for printing sectors

hw/block/xen: use proper format string for printing sectors

The %lu format string is different depending on the host architecture
which causes builds like the debian-armhf-cross build to fail. Use the
correct PRi64 format string.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190116121350.23863-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>