scmi: Prepare v0.3.0 release

[mz-pdm]

Update CHANGELOG.md and Cargo.toml for v0.3.0 release. This release adds SCMI notification support.

Summary of the PR

Let's make with a new release of vhost-device-scmi, with SCMI notification support.

Requirements

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

• [ x] 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.
• [ 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.

[stefano-garzarella]

I can do the release when we will receive a second approval ;-)

[stefano-garzarella]

crate published: https://crates.io/crates/vhost-device-scmi/0.3.0

[mz-pdm]

Thanks!