vswitch: Break out XenServer-specific external ids

As we move to new platforms, not all external identifiers will be
universally unique, but the "-uuid" suffix seemingly prevents their use.
Create new identifiers without the "-uuid" suffix.  Change the existing
XenServer-specific external identifiers to contain a "xs-" prefix.  This
also allows a XenServer integrator to define identifiers different from
the XenServer UUIDs, but still leave them in the config database to be
used by other programs.