Introduce this new field for the accelerator classes so that each
specific accelerator in the future can register its own global
properties to be used further by the system. It works just like how the
old machine compatible properties do, but only tailored for
accelerators.

Introduce register_compat_props_array() for it. Export it so that it may
be used in other codes as well in the future.
Suggested-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-3-git-send-email-peterx@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>