xen: add support for initializing xenstore later as HVM domain

When running as dom0less guest (HVM domain on ARM) the xenstore event
channel is available at domain creation but the shared xenstore
interface page only becomes available later on.

In that case, wait for a notification on the xenstore event channel,
then complete the xenstore initialization later, when the shared page
is actually available.

The xenstore page has few extra field. Add them to the shared struct.
One of the field is "connection", when the connection is ready, it is
zero. If the connection is not-zero, wait for a notification.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220513211938.719341-2-sstabellini@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>