Fix mispopulation of nic parameters at nic modify

There's a bug in Ganeti 2.1 rc0 that makes nic parameters be populated
from the "filled in" dict, even if we're not changing any values in
them. This patch fixes the problem, by populating them from the correct
(unfilled) dict.

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