vhost_user: Remove support for FS_* requests

[aesteve-rh]

Summary of the PR

Since these non-standard backend request message types are confirmed not to be used anymore, let's just remove them.

pub enum BackendReq {
    ...
    /// Virtio-fs draft: map file content into the window.
    FS_MAP = 100,
    /// Virtio-fs draft: unmap file content from the window.
    FS_UNMAP = 1001,
    /// Virtio-fs draft: sync file content.
    FS_SYNC = 1002,
    /// Virtio-fs draft: perform a read/write from an fd directly to GPA.
    FS_IO = 1003,
    ...
}

Closes: #213

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]

The best way to increase coverage, removing old code :)

[germag]

LGTM