Device custom properties: Keep non-custom devices in domxml

Bug fix for device custom properties feature: Previously only
devices with custom properties were added to the VM's domxml
because of stupidity. Now all devices are added and device
creation hooks are called only for devices with custom
properties.

Patch Sets 2-5:
Styling, naming.

Change-Id: Ifb4f9eb32bc58371cec66301275689660193447b
Signed-off-by: Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13637
Reviewed-by: Dan Kenigsberg <danken@redhat.com>