vdsm: expose hostdev via vdsClient

Exposes current API verb hostdevListByCaps by vdsClient,
using pretty tree graphical structure.

Change-Id: I1e4dd1a1ee9af0d1873a081c115289ad2e52204f
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32316
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>