Many Xen PV frontends share similar code for setting up a ring page
(allocating and granting access for the backend) and for tearing it
down.
Create new service functions doing all needed steps in one go.
This requires all frontends to use a common value for an invalid
grant reference in order to make the functions idempotent.
Changes in V3:
- new patches 1 and 2, comments addressed
Changes in V2:
- new patch 9 and related changes in patches 10-18