vhost: fix double-locking in Backend to Frontend request handlers

[stefano-garzarella]

Summary of the PR

Some methods acquire the mutex lock twice: once to check feature flags
and again when calling send_message(), this is inefficient and also
racy, so acquire the lock once and reuse the guard for both operations.
Remove send_message() wrapper method now unused.

The first 2 patches are just in preparation to clarify method and avoid
mapping errors all over the places.

This was initially discussed at: #251 (comment)

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• 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.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[epilys]

@stefano-garzarella LGTM, can you fix the commit length?

E AssertionError: For commit '51e27fe', title exceeds 60 chars. Please keep it shorter.
E assert 64 <= 60
E + where 64 = len('vhost: fix double-locking in Backend to Fronted request handlers')

[stefano-garzarella]

@stefano-garzarella LGTM, can you fix the commit length?

E AssertionError: For commit '51e27fe', title exceeds 60 chars. Please keep it shorter.
E assert 64 <= 60
E + where 64 = len('vhost: fix double-locking in Backend to Fronted request handlers')

sure, thanks for the reminder (I hate that rule xD)

[stefano-garzarella]

v3:

• removed node()/lock() completely [@epilys]
• fixed changelog entry [@epilys]

[epilys]

@stefano-garzarella weird error, is this even related to the changes?

thread 'vhost_kern::vsock::tests::test_vsock_ioctls' panicked at vhost/src/vhost_kern/vsock.rs:192:37:
called `Result::unwrap()` on an `Err` value: IoctlError(Os { code: 98, kind: AddrInUse, message: "Address already in use" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[stefano-garzarella]

@stefano-garzarella weird error, is this even related to the changes?

thread 'vhost_kern::vsock::tests::test_vsock_ioctls' panicked at vhost/src/vhost_kern/vsock.rs:192:37:
called `Result::unwrap()` on an `Err` value: IoctlError(Os { code: 98, kind: AddrInUse, message: "Address already in use" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

mmm, I already saw that, I think we have some little race between tests reusing the same vsock CID. I'll fix it, but yeah, it's unrelated. Let me restart them.