virtio_pci: move structure to a header

These are guest/host interfaces, so they belong in the header where e.g.
qemu will know to find them.

Note: we added a new structure as opposed to extending existing one
because someone might be relying on the size of the existing structure
staying unchanged.  Add a warning to avoid using sizeof.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

include/uapi/linux/virtio_pci.h

@@ -166,6 +166,17 @@ struct virtio_pci_common_cfg {
 	__le32 queue_used_hi;		/* read-write */
 };
 
+/*
+ * Warning: do not use sizeof on this: use offsetofend for
+ * specific fields you need.
+ */
+struct virtio_pci_modern_common_cfg {
+	struct virtio_pci_common_cfg cfg;
+
+	__le16 queue_notify_data;	/* read-write */
+	__le16 queue_reset;		/* read-write */
+};
+
 /* Fields in VIRTIO_PCI_CAP_PCI_CFG: */
 struct virtio_pci_cfg_cap {
 	struct virtio_pci_cap cap;