Remove non-standard BackedReq types

[germag]

The following back-end request types are not part of the vhost-user protocol and should be removed:

pub enum BackendReq {
    ...
    /// Virtio-fs draft: map file content into the window.
    FS_MAP = 6,
    /// Virtio-fs draft: unmap file content from the window.
    FS_UNMAP = 7,
    /// Virtio-fs draft: sync file content.
    FS_SYNC = 8,
    /// Virtio-fs draft: perform a read/write from an fd directly to GPA.
    FS_IO = 9,
    ...
}

These are not in the vhost-user standard, which in itself is confusing, but even conflict with values that are defined in the standard (see Back-end message types), such as: VHOST_USER_BACKEND_SHARED_OBJECT_ADD, VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE and VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP.

This issue is to continue the discussion started on #205