Add ability to set mode=private to a macvlan interface.

[farsonic]

As discussed in in open issues this is a pull-request to add mode=private to a MACVLAN interfaces, completing all current available options for this type of interface.

Summary by CodeRabbit

• New Features

  • Introduced a new "private" network interface mode that enables users to configure virtual machines with private networking options.

• Documentation

  • Updated documentation to explain the behavior of private networking, including how packets are routed through external bridges and routers under this mode.

Description by Korbit AI

What change is being made?

Add support for setting a macvlan interface to mode 'private' in the libvirt domain configuration.

Why are these changes being made?

Previously, the libvirt provider did not support the 'private' mode for macvlan interfaces, preventing users from configuring this networking mode for increased isolation and security within the same host environment. This change addresses the need for more versatile network configuration options by enabling the use of 'private' mode, ensuring that packets are only delivered to virtual machines on the same host through an external router or gateway, if applicable.

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[korbit-ai]

I've completed my review and didn't find any issues...

[dmacvicar]

Just for my own context in the future:

The network_interface block in our schema is flattened. This means vepa, macvtap, passthrough represent a device, which then gets converted to libvirt source.direct.dev in all cases, with a differerent mode.

I don't like this, but it is not something we will change in the domain schema (may be a v2).

Adding a private one just follows this approach and should be harmless.

[dmacvicar]

Thanks for the contribution 🙏