virtio-bindings: prepare 0.3.0 release

[Ablu]

Requirements

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

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

[vireshk]

Should we get this via https://github.com/rust-vmm/vm-virtio/pull/250 instead ?

[vireshk]

I have already made this v0.2.1 with my pull request, as @jiangliu suggested.

[Ablu]

Ah. Did not realize that you needed an virtio-bindings release for your patches too :)