Clean master slave

[aesteve-rh]

Summary of the PR

Change uses of master/slave for backend/frontend in the codebase.

Requirements

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

• All commits in this PR are signed (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.

[sboeuf]

LGTM

[stefano-garzarella]

I left some minor comments, but it LGTM ;-)

Since it is a breaking change, @jiangliu @eryugey @slp do you have any thought on this?

[aesteve-rh]

Since it is a breaking change

Yeah... I can help to update other repositories consuming this API that would be broken after the update.
What would be the best way/order to proceed for those?

[stefano-garzarella]

Yeah... I can help to update other repositories consuming this API that would be broken after the update.

Thanks! Really appreciated!

What would be the best way/order to proceed for those?

I think we need to publish a new release of vhost and vhost-user-backed as a first step. Since both are still 0.y.z, a new y release should be fine since it is a major release. Then we can start to update dependencies like vhost-device's crates, virtiofsd, etc.

[aesteve-rh]

@stefano-garzarella thanks!

I'll keep an eye on the next release and start updating other repositories once available (probably will start with vhost-device, as I already have a fork).