webadmin: Attach disk - verify valid selection

Fix validation that at least one disk
is selected on attach disk.

Change-Id: Idde1f85c3b1a8d31fc6a5ddd6591f493dc113b3d
Bug-Url: https://bugzilla.redhat.com/879218
Signed-off-by: Daniel Erez <derez@redhat.com>