plat/virtio: Implement support for event index notification suppression

plat/virtio: Implement support for event index notification suppression

This allows the drivers/device to specify the other side should
send a notification. This is required for firecracker which does not
support the original notification suppression flag.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #1089