CA-107425: Fixing the loss of default route when executing network tests

The series of commits in this pull request are to fix a bug found whereby the default route of the host is modified.

Because the test kit assigns IPs to interfaces in Dom0, usually via DHCP, the default route may sometimes change.

The patches allow us to take a snapshot at the first time we execute the test kit, and then everytime we action a cleanup, we will make sure that the default route re-added.