ioctls: Add new MshvError type derived from thiserror::Error

[NunoDasNeves]

MshvError allows different kinds of errors to be returned from the
library. One intended use is to return details about a failed hypercall
from the MSHV_ROOT_HVCALL ioctl, so useful error messages can be
printed.

Implement from_hvcall for future use with MSHV_ROOT_HVCALL, along with
tests.

Use into() to convert uses of errno::Error to MshvError.

Summary of the PR

This is a precursor patch to #128.

The only way to debug hypercall failures that result from the IOCTL calls in mshv-ioctls today, is to use dmesg and rely on debug printing in the kernel itself.

The MshvError type is the first step to solving thism this by allowing mshv-ioctls to return either an errno::Error or details about a hypercall-related error (hypercall code and status code).

This will help users/developers of VMMs identify the root cause of issues more reliably.

This PR introduces a couple of dependencies:

• https://crates.io/crates/thiserror is used in many places in rust-vmm to create error enums with automatically generated From and Display traits. Similarly, here it is used to define MshvError.
• https://crates.io/crates/num_enum is the defacto way to convert integers to enums by implementing the TryFromPrimitive trait. It is used to convert u16 to enum HvError. If there is a better way to do this please let me know!

Since this PR changes the Result type used by mshv-ioctls, it may cause API breakages with other crates. From testing with Cloud Hypervisor, I have only found one, in the vfio-ioctls crate. It is fixed in a backward-compatible manner here: rust-vmm/vfio#44

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.

[NunoDasNeves]

NOTE: All the changes are in the 2nd commit. The first is just cargo fmt replacing all the newlines in hvdef.rs.