frontend: Fix disk creation NPE on diskprofile selection

I was getting a NullPointer Exception when adding disks in oVirt 3.5
prerelease.

Bug-Url: https://bugzilla.redhat.com/1149442
Change-Id: Ia3338051ae3888fb672419c6a4a9cb1cc03e6fde
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>