Fix bug in instance net changes

_PrepareNicModification returns the invalid type, which triggers an
assert resulting in a mysterious error:

Failure: command execution error:

Without any explanation. We fix this by removing the return value from
_PrepareNicModification, and instead returning the expected type
(since it differs per create/modification) from the (existing)
wrappers for this function. We don't need to return actual changes
from this function as _ApplyNicMods is the function that
computes/returns the formatted changes.

Signed-off-by: Iustin Pop <iustin@google.com>
Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>