argo: implement the unregister op

argo: implement the unregister op

Takes a single argument: a handle to the ring unregistration struct,
which specifies the port and partner domain id or wildcard.

The ring's entry is removed from the hashtable of registered rings;
any entries for pending notifications are removed; and the ring is
unmapped from Xen's address space.

If the ring had been registered to communicate with a single specified
domain (ie. a non-wildcard ring) then the partner domain state is removed
from the partner domain's argo send_info hash table.

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