xenapi: Use netuils.get_injected_network_template

The Xen driver duplicates quite a bit of code used to inject the `interfaces`
template into the guest.

This patch makes the Xen driver use the generic `netutils` version.

Parameter defaults were moved from the signature into the method since in the
original code, you wouldn't be able to override them due to their value being
populated at import-time, not run-time.

Change-Id: I4cd1631b8e6ec2dc3fae1f8eb0362aecead53f1e
Merge "xenapi: Use netuils.get_injected_network_template"