Per device custom properties for before/after vm destroy.

Patch Set 2:
Run before/after device destroy hooks only on devices that
have custom properties. after_device_create,
before_device_destroy and after_device_destroy hook calls
now share code (Removed code duplication).

Patch Set 3+:
Code styling and prettifying.

Change-Id: Ic18b838dc51ea6bde31ef1a4bf0d0d372d95cd41
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13411
Reviewed-by: Dan Kenigsberg <danken@redhat.com>