Prepare the release of virtio-queue v0.12.0 and virtio-queue-ser v0.9.0

[stefano-garzarella]

Summary of the PR

This release contains the new Reader/Writer classes to iterate over descriptors. This is useful for vhost devices (see https://github.com/rust-vmm/vhost-device/pull/580).

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]

@epilys I can do that, but we are not changing any previous API used by that crates with this release, so I don't know. In addition, I saw that for previous releases we are updating the changelog only when doing the release.

[epilys]

@stefano-garzarella ah ok, makes sense to me!