Updating README with vdsm-tool new api

Change-Id: I9c2f06cbb48030c1340e1a1738149c7c62e66460
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22910
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>