vdsm-tool: simplify getting modules by names.

Change-Id: Idb056e18573b1aadefe57a6eee4ec1396dad9775
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31464
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>