hostdev: make caps in listByCaps truly optional

In order to make the api cleaner, removes the need to specify the
caps parameter (defaulting to None which means all devices)

Change-Id: Ifc4f5ff217a12f0ffe3d75eec1e6c25c6d211fd7
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35867
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>