Remove KVM provider and use LXD --vm

[Simon Richardson]

Given that LXD now has support for virtual machines[1], we should converge the code path to use LXD for containers and vms. We can dogfood our own tooling and give better feedback to tooling et al.

1. https://discuss.linuxcontainers.org/t/running-virtual-machines-with-lxd-4-0/7519

[Stéphane Graber (stgraber)]

Related to https://bugs.launchpad.net/juju/+bug/1897746

On our recent call, the suggestion was to use the existing `virt-type` constraint and extend it to the lxd provider with two supported values, "container" (default) and "vm".

Juju could then start making use of that instead of the kvm provider.

[Erik Lönroth (erik-lonroth)]

To be able to leverage LXD capability to spawn instaces with their own kernel is a game changer for many situations.

I think this feature is massive.

[Simon Déziel (sdeziel)]

@erik-lonroth, it's now possible to ask Juju (3.1.0) to spin up VMs. This was added by Simon Richardson via https://github.com/juju/juju/pull/15004. I just tested it and it works!

I'd guess that removing the KVM provider is the only remaining thing to do for this bug.