vsock: Set VhostUserProtocolFeatures::MQ

[vireshk]

As per the vhost-user specification:

"Back-ends should always implement the VHOST_USER_PROTOCOL_F_MQ protocol feature, even for devices with a fixed number of virtqueues, since it is simple to implement and offers a degree of introspection."

Vsock backend support three virtqueues and hence this should be set.

Summary of the PR

Please summarize here why the changes in this PR are needed.

Requirements

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• [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.
• [ ] 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]

LGTM!

@vireshk the audit warning is fixed in the current main, so I guess a rebase should fix it.

[stefano-garzarella]

@epilys thanks for the changelog entry!