xen/store: do not store local values in xen_start_info

There's no need to store the xenstore page or event channel in
xen_start_info if they are locally initialized.

This also fixes PVH local xenstore initialization due to the lack of
xen_start_info in that case.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>