Import katapult_virtual_machine resources

[jimeh]

We currently do not support importing katapult_virtual_machine resources, as
the disk_template and disk_template_options attributes are only relevant
during creation, and are not returned by the API at the moment.

There are plans for the API to return these attributes, but it's not a straight
forward thing, so we probably should not delay import support until it's ready.

Based on the import documentation, we probably want to use
ImportStateVerifyIgnore, and simply require users to define a lifecycle
block with a ignore_changes attribute to avoid the disk template options from
showing as changed after import.

It might be possible to use CustomizeDiff to suppress changes to
attributes which are only available during create. I did have a very brief look
into this back in December, but didn't have much luck, so I'm not sure if it's
possible.

[jimeh]

With the planned migration in #97 to the new Plugin Framework, along with some purpose made enhancements to Katapult's Core API, this issue should become easier to resolve.

[verenion]

+1

The ability to import resources is something that is blocking us managing all of our resources in Terraform.