vhost-user-backend: prepare v0.13.1 release

[stefano-garzarella]

Summary of the PR

Fix a regression introduced by commit 7f326dd ("vhost-user-backend: SET_FEATURES must not disable any ring").

SemVer compatibility checked:

$ cargo semver-checks check-release -p vhost-user-backend
     Parsing vhost-user-backend v0.13.1 (current)
     Parsing vhost-user-backend v0.13.0 (baseline, cached)
    Checking vhost-user-backend v0.13.0 -> v0.13.1 (minor change)
   Completed [   0.030s] 42 checks; 42 passed, 6 unnecessary

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.