engine: Use proper method to mark net changes as saved

The patch reduces the need for updating the entire VdsDynamic
data by using a designated method for saving the dirty-network
indicator of the host.

Change-Id: Ib4c54214a275a1653c46bca7c95ff847975dd7d4
Signed-off-by: Moti Asayag <masayag@redhat.com>