Update vmm-sys-util to 0.12.1 and prepare 0.14.0 release

[roypat]

The vmm-sys-util update brings a fix for a security vulnerability behind feature-gated code not used by vm-memory (the with-serde feature), see https://github.com/advisories/GHSA-875g-mfp6-g7f9

Requirements

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

• [ ] 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.
• [ ] 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.

[roypat]

This is blocking https://github.com/rust-vmm/vhost/pull/220