restapi : add descriptions to import vm/template SD source/destination params in RSDL

In import vm/template action from cli there is no
description indicating which storagedomain is the
source storagedomain and which one is the destination.

Change-Id: Ie1f5e28d5c3d8047e39c8d3f0fbe95a794c2d9dc
Bug-Url: https://bugzilla.redhat.com/960997
Signed-off-by: Ravi Nori <rnori@redhat.com>