block: removed virtio flush feature

[georgepisaltu]

Reason for This PR

The flush operation of the block file is actually a noop and the feature is therefore incorrectly offered to the guest driver.

Description of Changes

Removed the virtio flush feature from the block device.

• This functionality can be added in rust-vmm.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any newly added unsafe code is properly documented.
• Any API changes are reflected in firecracker/swagger.yaml.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[sandreim]

This is something worthy of a changelog entry 😄

[georgepisaltu]

This is something worthy of a changelog entry 😄

I anticipate we will add the feature back before the next release when we find a way to correctly implement it, but I can temporarily add it to the changelog as well.

[acatangiu]

This is something worthy of a changelog entry 😄

I anticipate we will add the feature back before the next release when we find a way to correctly implement it, but I can temporarily add it to the changelog as well.

Yes, keep this PR/commit complete. When we add the feature back we'll probably do that by a revert commit which will work best if we keep things tight.

[georgepisaltu]

Closing this due to a new course of action, detailed here.