Added support for before and after device create.

before_device_create is called for each device,
that has custom properties, right before
before_vm_create. Each such hook is supplied the device's
dom xml and any custom properties. after_device_create is
called for each device that has custom properties
right after after_vm_start.

Patch Set 2:
Changed it so only devices that have custom properties
call such per-device hooks. Styled up code.

Patch Set 3+
Styling / fixing comments.

Change-Id: I3291032ff85254d5c3a4260bffb7003ca3502d22
Signed-off-by: Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13396
Reviewed-by: Dan Kenigsberg <danken@redhat.com>