Fixes #32083 - use full Host model in VM template

For rendering `hosts/_compute` we've used just host attributes in the
past, but it just brings more complexity, when we cant rely on the Host
model to be around.

This builds the Host model instead and passes it to the templates.

Fixes #32084 - document default attributes retrieve

Documenting how we are currently retrieving default attributes for an
new ComputeResource VM instace. Meaning how we render VM tab for new
Host.