Add shared object

[aesteve-rh]

Summary of the PR

Add SHARED_OBJECT_* vhost-user message IDs to align with standard at https://qemu-project.gitlab.io/qemu/interop/vhost-user.html

Move FS_* type IDs to avoid current and future collisions.

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.

[aesteve-rh]

v3 -> v4: (I think) Separate the frontend_req_handler test fix (joining the frontend handler threads) to its own commit.

[stefano-garzarella]

Thanks, LGTM!

Last doubt: whether we should merge commit 3 and 4 together to improve bisectability.

[aesteve-rh]

Last doubt: whether we should merge commit 3 and 4 together to improve bisectability.

I do not have a strong opinion either way. Commit 4 updated BackendInternal and vhost-user-backend, and did not change any parts of the shared object logic, except for the check itself. So it felt a contained feature that would be clearer to review in a separate commit. On the other hand, it is still part of the shared object support, so I see your point.

If in doubt, I can squash them as they are already reviewed anyway.

[stefano-garzarella]

Last doubt: whether we should merge commit 3 and 4 together to improve bisectability.

I do not have a strong opinion either way. Commit 4 updated BackendInternal and vhost-user-backend, and did not change any parts of the shared object logic, except for the check itself. So it felt a contained feature that would be clearer to review in a separate commit. On the other hand, it is still part of the shared object support, so I see your point.

If in doubt, I can squash them as they are already reviewed anyway.

I don't have a strong opinion either. Theoretically, patch 3 without patch 4 is incorrect, but only if the backend misbehaves. Also, the frontend should handle this eventually, so I think it's fine unless others have different opinions.

[aesteve-rh]

Also, the frontend should handle this eventually

I was so convinced that QEMU did this already… But I checked and, no, verifies it only on the backend side. You are right, commit 3 is incorrect by itself.

[stefano-garzarella]

My approval is still up :-) any other reviews? @Ablu @eryugey @germag @jiangliu @sboeuf @slp