Deploy KVM hosts with LXD by default

[webteam-app]

Bug originally filed by ack at https://bugs.launchpad.net/bugs/1922876

The backend now supports deploying VM hosts with LXD in addition to libvirt.

In the UI we want to make this the default when selecting the "deploy as kvm host" checkbox.

For this, the register_vmhost flag should be set to true instead of install_kvm.

[Caleb-Ellis]

@amylily1011 any thoughts on how to handle this? I was thinking once the KVM checkbox is checked we show a radio selection for LXD (default) and libvirt? So something like:

[amylily1011]

Oh, this is a great idea, put LXD on default 🔥 let's do it!