The default for GUEST_INTERFACE_DEFAULT now uses the ip command
to find an interface; so it will work on multiple distributions.
XenAPI should not be setting a specific interface here, as it will
almost always be wrong. In most cases, the calculated value for
GUEST_INTERFACE_DEFAULT will be a better default.
PUBLIC_INTERFACE_DEFAULT makes even less sense as it's often an
internal bridge for devstack scenarios.
In both cases, the right way to override these is to set
GUEST_INTERFACE / PUBLIC_INTERFACE in the localrc rather than
changing the _DEFAULT values.
Change-Id: I0cf84438d778bf1a2481328165513c59167490e2
c694e55ad9