r/virtual_machine: Device cull range fix, and handle zero-NIC templates

[vancluever]

There was some issues with our device culling/ranging logic in certain
scenarios:

• On templates with no NICs, unitRange was not producing a proper
  result. This was the result of these values being pointers because we
  work with the unit number value directly in the virtual device, which is
  a pointer. This fixes things so that if a zero-length device list is
  passed to unit range, we return 0 immediately (as there are no devices
  to range over).
• On templates where the NIC count was less than the amount of NICs
  needed by configuration, the device cull range expression (which is used
  to handle the exact opposite scenario) was running into out-of-range
  issues. This has been fixed by asserting that the old device list is
  indeed longer than the new device list so that there's no way that
  referencing old[len(new):] will fail. This has been extended to CD
  devices too, which has the same logic, even though it's currently masked
  by the fact that we only allow one cdrom device.

Fixes #267.

[vancluever]

Just waiting for some clone tests to finish and will add the results when hey are done.

[vancluever]

Test results:

--- PASS: TestAccResourceVSphereVirtualMachine/clone_from_template (125.25s)
--- PASS: TestAccResourceVSphereVirtualMachine/clone,_multi-nic_(template_should_have_one) (120.24s)
--- PASS: TestAccResourceVSphereVirtualMachine/clone_with_different_timezone (127.92s)
--- PASS: TestAccResourceVSphereVirtualMachine/clone_with_bad_timezone (0.01s)
--- PASS: TestAccResourceVSphereVirtualMachine/clone_with_different_hostname (120.12s)
--- PASS: TestAccResourceVSphereVirtualMachine/clone_with_extra_disks (294.94s)

[mbfrahry]

LGTM!

[vancluever]

Thanks @mbfrahry!