frontend: do not update nics which did not changed

Edit VM dialog -> if the nic->network assignment did
not change do not send the UpdateVmInterface - do
it only for the changed nics.
This logic has already been present for the create VM
from template, so generalized it.

Change-Id: I11e32d013acf9fbf8dd3623d051403721786690b
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>
Bug-Url: https://bugzilla.redhat.com/988043