Revert "virtqueue: Try to further reduce unnecessary interrupts"
Revert "virtqueue: Try to further reduce unnecessary interrupts"

This reverts commit c6554e9c7e9a8f07aae6ded2d9033c6c1a7d4923.

Reason for revert:

`time tast run mycros arc.Boot.vm` will always halt at

```
  Waiting for Android boot
  Waiting for initial ARC process
```
till timeout.

Original change's description:
> virtqueue: Try to further reduce unnecessary interrupts
>
> Try to reduce unnecessary interrupts by updating `signalled_used` every
> time we check `used_event` rather than only when we actually write to
> the eventfd.
>
> This technique is lifted out of the vhost_add_used_n and vhost_notify
> methods in drivers/vhost/vhost.c in the kernel and the variable names
> are changed to more easily compare the two implmentations.
>
> This combined with the other notification suppression changes give a
> performance improvement of ~10% across all storage devices in the
> blogbench benchmark.
>
> BUG=none
> TEST=vm.Blogbench.{p9,block,virtiofs}
>
> Change-Id: I618767e4c36ecae607b7641e54a029c25583844d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3026691
> Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
> Commit-Queue: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
> Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

BUG=b:194452080
TEST=time tast run mycros arc.Boot.vm

Change-Id: Ib7511dec3e0df26be54616f20226ded598fc8a37
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3051292
Bot-Commit: Rubber Stamper <d516c4cc2192058ab8dae3d306acbdaf2ce6574f@appspot.gserviceaccount.com>
Auto-Submit: Chih-Yang Hsia <e09ac2debfefa3666e69a4e41e179a1517e8fddf@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm