vdsm-tool: prefix private methods and properties with _

Change-Id: I660c5492af2ed5e3ffb61b0a5308a43e03a0ce93
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27821
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>