Fix off-by-one error when modifying instance NIC

For an instance with exactly one NIC:

$ gnt-instance modify --net 1:ip=1.2.3.4 inst1
Failure: prerequisites not met for this operation:
error type: wrong_input, error details:
Invalid NIC index 1, valid values are 0 to 1

For an instance with no NIC at all, it fails with “Invalid NIC index 0, valid
values are 0 to 0”. This is fixed by this patch.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>