Use a global variable to store the start flags for both PV and PVH.
This allows the xen_initial_domain macro to work properly on PVH.

Note that ARM is also switched to use the new variable.
Signed-off-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: NRoger Pau Monné <roger.pau@citrix.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NJuergen Gross <jgross@suse.com>