engine: Ties more consistently between vm nic device and vm nic entities.

Fixes copy of vm nic devices in the same way the
http://gerrit.ovirt.org/#/c/17854/ fixes the copy of disk devices.

Change-Id: Iee9267ade386ab83a6c353b4206f4dfe003f24bc
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>