hw/virtio/virtio-pmem: Replace impossible check by assertion

hw/virtio/virtio-pmem: Replace impossible check by assertion

The get_memory_region() handler is used when (un)plugging the
device, which can only occur *after* it is realized.

virtio_pmem_realize() ensure the instance can not be realized
without 'memdev'. Remove the superfluous check, replacing it
by an assertion.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-Id: <20231017140150.44995-2-philmd@linaro.org>