build(deps): update nix requirement from 0.26 to 0.27

Updates the requirements on [nix](https://github.com/nix-rust/nix) to permit the latest version.
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](nix-rust/nix@v0.26.0...v0.27.1)

---
updated-dependencies:
- dependency-name: nix
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

nix 0.27 required these changes:
- memfd_create() is now only available with crate feature `fs`.
- the return value of memfd_create() is now an OwnedFd and not RawFd
  anymore.

We are using nix only for testing, so no functional changes at all.

Signed-off-by: Stefano Garzarella <[email protected]>

Author: dependabot[bot]

crates/vhost-user-backend/Cargo.toml

@@ -21,7 +21,7 @@ vm-memory = { version = "0.12.0", features = ["backend-mmap", "backend-atomic"]
 vmm-sys-util = "0.11.0"
 
 [dev-dependencies]
-nix = "0.26"
+nix = { version = "0.27", features = ["fs"] }
 vhost = { path = "../vhost", version = "0.8", features = ["test-utils", "vhost-user-frontend", "vhost-user-backend"] }
 vm-memory = { version = "0.12.0", features = ["backend-mmap", "backend-atomic", "backend-bitmap"] }
 tempfile = "3.2.0"

crates/vhost-user-backend/tests/vhost-user-server.rs

@@ -1,7 +1,7 @@
 use std::ffi::CString;
 use std::fs::File;
 use std::io::Result;
-use std::os::unix::io::{AsRawFd, FromRawFd};
+use std::os::unix::io::AsRawFd;
 use std::os::unix::net::UnixStream;
 use std::path::Path;
 use std::sync::{Arc, Barrier, Mutex};
@@ -153,8 +153,7 @@ fn vhost_user_client(path: &Path, barrier: Arc<Barrier>) {
         nix::sys::memfd::MemFdCreateFlag::empty(),
     )
     .unwrap();
-    // SAFETY: Safe because we panic before if memfd is not valid.
-    let file = unsafe { File::from_raw_fd(memfd) };
+    let file = File::from(memfd);
     file.set_len(0x100000).unwrap();
     let file_offset = FileOffset::new(file, 0);
     let mem = GuestMemoryMmap::<()>::from_ranges_with_files(&[(