Add support for generic notifier lists

Add support for generic notifier lists

Notifiers are data-less callbacks and a notifier list is a list of registered
notifiers that all are interested in a particular event.

We'll use this in a few patches to implement mouse change notification.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
[cherry-picked from d1e70c5e6d1472856c52969301247fe8c3c8389d
    conflicts: used the sys-qeue interface and added required
    LIST_REMOVE_SAFE function to that]

Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>