VirtualDomain: Log domain status in debug mode for xl and xen-list

When checking the status of a Xen domain (with either xl or xen-list), $OCF_SUCCESS should not be returned directly. Rather, rc should be set to the correct status, so the following message can be reported in the debug log:

   "Virtual domain $DOMAIN_NAME is currently (running|not running)"

Also, only try xen-list if xl does not exist.