vfio-pci: Add device request interface

Userspace can opt to receive a device request notification,
indicating that the device should be released.  This is setup
the same way as the error IRQ and also supports eventfd signaling.
Future support may forcefully remove the device from the user if
the request is ignored.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

include/uapi/linux/vfio.h

@@ -333,6 +333,7 @@ enum {
 	VFIO_PCI_MSI_IRQ_INDEX,
 	VFIO_PCI_MSIX_IRQ_INDEX,
 	VFIO_PCI_ERR_IRQ_INDEX,
+	VFIO_PCI_REQ_IRQ_INDEX,
 	VFIO_PCI_NUM_IRQS
 };