engine: Sending to the host oVirt-level default MTU

The concept of "MTU: Host's default" is eliminated.
In case there is no MTU value on the Network, oVirt-level default MTU will
be sent to the host.

Change-Id: Ice358e3f4c10b08b9ddaed3b965d26e62cdb558a
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>