iface: Consistent order on serialization

When state contains OVS interface and OVS bridge sharing the same name,
the JSON/YAML serialization produces random ordering on these two
interfaces.

To produce consistent interface ordering, the `Interfaces::to_vec()`
function will sort with `InterfaceType` as second sort consideration.

Unit test case included.

Resolves: https://issues.redhat.com/browse/RHEL-130157

Signed-off-by: Gris Ge <fge@redhat.com>