vm: Fix custom device properties on hot operation

On hot plug & unplug of a vNIC/vDisk the custom device parameters should
be taken from the device itself, as is done in the run VM operation,
thus keeping a consistent API.

Change-Id: I1f1c1b620cbcf5538bd0fac8c564952153977398
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15987
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>
Tested-by: Assaf Muller <amuller@redhat.com>