hooks: Add disk hot plug/unplug hooks

Added four hook points named:
1. before_disk_hotplug
2. after_disk_hotplug
3. before_disk_hotunplug
4. after_disk_hotunplug

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=908656
Change-Id: I10b689964fad3a1118974f7ba7b91e795cbffd1f
Signed-off-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12479
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>