tools: init.d/Linux/xencommons: run script only when needed

tools: init.d/Linux/xencommons: run script only when needed

Currently xencommons prints an error that /proc/xen/capabilities does
not exist when started on a non-xen kernel.

Update the xencommons script to run only when needed:
- do not run if /proc/xen does not exist
- check if /proc/xen/capabilities exists before doing the grep for dom0

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>