xenbus: save xenstore local status for later use

Save the xenstore local status computed in xenbus_init. It can then be used
later to check if xenstored is running in this domain.

Signed-off-by: Aurelien Chartier <aurelien.chartier@citrix.com>
[Changes in v4:
- Change variable name to xen_store_domain_type]
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>