virt-qemu: Add back nic script/downscript

Initialy thought this was disused.  However, original implementation
would duplicates path mangling in multiple locations (vm.create and
vm.add_nic).  Since this is a property that follows nic params, it makes
more sense to add it as property along with path mangling to the
storage class directly.

Signed-off-by: Chris Evich <cevich@redhat.com>