xe-toolstack-restart: Handle xenopsd proliferation, and only restart enabled services

We now have multiple xenopsd processes for different backend libraries.
The restart script needs to handle this, restarting only the variant
that is supposed to be running.

This change is more general, and only restarts any Xapi-related process
if chkconfig says that it should be running.

Signed-off-by: Euan Harris <euan.harris@citrix.com>