virtio: define VIRTIO_PCI_CAP_PCI_CFG in header.

This provides backdoor access to the device MMIOs, and every device should
have one.  From the virtio 1.0 spec (CS03):

  4.1.4.7.1 Device Requirements: PCI configuration access capability

  The device MUST present at least one VIRTIO_PCI_CAP_PCI_CFG capability.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

include/uapi/linux/virtio_pci.h

@@ -109,8 +109,10 @@
 #define VIRTIO_PCI_CAP_NOTIFY_CFG	2
 /* ISR access */
 #define VIRTIO_PCI_CAP_ISR_CFG		3
-/* Device specific confiuration */
+/* Device specific configuration */
 #define VIRTIO_PCI_CAP_DEVICE_CFG	4
+/* PCI configuration access */
+#define VIRTIO_PCI_CAP_PCI_CFG		5
 
 /* This is the PCI capability header: */
 struct virtio_pci_cap {