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>