virtio: define virtio_pci_cfg_cap in header.

We already have VIRTIO_PCI_CAP_PCI_CFG, let's define the structure that
goes with it.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/uapi/linux/virtio_pci.h

@@ -157,6 +157,12 @@ struct virtio_pci_common_cfg {
 	__le32 queue_used_hi;		/* read-write */
 };
 
+/* Fields in VIRTIO_PCI_CAP_PCI_CFG: */
+struct virtio_pci_cfg_cap {
+	struct virtio_pci_cap cap;
+	__u8 pci_cfg_data[4]; /* Data for BAR access. */
+};
+
 /* Macro versions of offsets for the Old Timers! */
 #define VIRTIO_PCI_CAP_VNDR		0
 #define VIRTIO_PCI_CAP_NEXT		1