argo: implement the notify op

argo: implement the notify op

Queries for data about space availability in registered rings and
causes notification to be sent when space has become available.

The hypercall op populates a supplied data structure with information about
ring state and if insufficient space is currently available in a given ring,
the hypervisor will record the domain's expressed interest and notify it
when it observes that space has become available.

Checks for free space occur when this notify op is invoked, so it may be
intentionally invoked with no data structure to populate
(ie. a NULL argument) to trigger such a check and consequent notifications.

Limit the maximum number of notify requests in a single operation to a
simple fixed limit of 256.

Signed-off-by: Christopher Clark <christopher.clark6@baesystems.com>
Tested-by: Chris Patterson <pattersonc@ainfosec.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>