vhost-user-backend: prepare v0.13.0 release

[stefano-garzarella]

Summary of the PR

Before this release, vhost-user devices using this crate were limited to 32 memslots. This does not support well the integration with virtio-mem. For more details: https://issues.redhat.com/browse/RHEL-15317

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.

[stefano-garzarella]

@Ablu @sboeuf new release ready :-)