ovf: allow empty image description

Change-Id: I05558503bcbe99d7b35087da5703c0dd5655944f
Bug-Url: https://bugzilla.redhat.com/1147468
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>
(cherry picked from commit e2189302509bd2058160a8ccdd5b9b4a2e6b82b0)