medium: Xen: VirtualDomain: Fix issues with using the xl toolset for Xen (boo#901453)

Under xl, state and status are not returned with a --long listing. Instead,
the return code from `xl list $domain` must be used to determine if a domain
is running on the node or not.

When checking pid_status for domains running under Xen, use xl first. If not
available, use xen-list.