Added function get_enabled to export vring enable status.

[WeiChungHsu]

Summary of the PR

The vring is disabled and RX/TX queue is set to
not-ready during snapshot. But vhost-device process
keeps access vring and queue after snapshot which
caused system hang. We need to export vring enabled
and disabled status for snapshot feature so that
the vhost-device would skip vring/queue access
after snapshot to avoid hang.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[stefano-garzarella]

@WeiChungHsu why closing #284 in favor of this? What are the differences?

Please in the future avoid that, it's better to continue on the same PR to check the history of reviews, etc.

[WeiChungHsu]

@WeiChungHsu why closing #284 in favor of this? What are the differences?

Please in the future avoid that, it's better to continue on the same PR to check the history of reviews, etc.

Sorry, I tried many git commands, but can't rebase this PR on current main. In addition, I accidentally close the old PR. Sorry about that.

[stefano-garzarella]

@WeiChungHsu why closing #284 in favor of this? What are the differences?
Please in the future avoid that, it's better to continue on the same PR to check the history of reviews, etc.

Sorry, I tried many git commands, but can't rebase this PR on current main. In addition, I accidentally close the old PR. Sorry about that.

No need to apologize, it can happen, it was to understand the reason, next time explain it even if it is a mistake.

[stefano-garzarella]

There are questions still open like: #284 (comment)

Furthermore, it hang because this Queue iter() operation return error at this line:
https://github.com/rust-vmm/vm-virtio/blob/main/virtio-queue/src/queue.rs#L592-L597

Hm. But shouldn't it error out in that case versus hanging? I do not fully understand where the hang would happen.

Ideally, I think it would be great if we can avoid easy footguns by designing the API in a way that avoids them altogether. > I assume chances are that people will forget this check in other implementations too?

[bilelmoussaoui]

This should be named is_enabled.