backends/none: Correctly check for existence.

We now set up a vm_id, so on "nixops check" we can properly show whether
the target machine exists or not.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
backends/none: Provide a resource ID value.

The value is the vm_id we generated on create() and currently is more or
less just for consistency and to check whether the vm_id exists on
"nixops info".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>