Add state migration for vm.vcpus

[pascal-hofmann]

The type change from string to float for vm.vcpus introduced with #92 breaks existing state if vcpus in the existing state is "":

│ Error: Failed to decode resource from state
│
│ Error decoding "netbox_virtual_machine.this" from previous state: a number is required

This PR bumps the schema version for resourceNetboxVirtualMachine to 1 and introduces a StateUpgrader that will gracefully upgrade the state, even if the value already is a float.

Further reading:

• https://www.terraform.io/plugin/sdkv2/resources/state-migration#resources-state-migration

[fbreckle]

Hi!

Thank you very much for this great PR.
Note that this is the first time I come in contact with state migrations, so bear with me on this one :)
I noticed that you do not throw errors under any circumstances. Is that a best practice pattern to not block the workflow for the user? The type signature of StateUpgradeFunc explicitly allows returning errors.

PS: The website link in your GitHub profile is unreachable ;)

[pascal-hofmann]

This is my first state migration too, but I think handling it gracefully is the way to go. Otherwise users would have to edit and fix state manually.

The website link in your GitHub profile is unreachable ;)

This is a known issue. 😬🙈