virtio_pmem: correct tags for config space fields

Since this is a modern-only device,
tag config space fields as having little endian-ness.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>

include/uapi/linux/virtio_pmem.h

@@ -15,8 +15,8 @@
 #include <linux/virtio_config.h>
 
 struct virtio_pmem_config {
-	__u64 start;
-	__u64 size;
+	__le64 start;
+	__le64 size;
 };
 
 #define VIRTIO_PMEM_REQ_TYPE_FLUSH      0