webadmin: improve warning on SPICE Native usb migration support

Webadmin will warn when choosing SPICE and native USB support only when
the cluster version is less than 3.2.

modified the DEFAULT EN message for that only.

Change-Id: I87b78ac7ef930b955ca70bc91303c4c083fc799c
Bug-Url: https://bugzilla.redhat.com/889551
Signed-off-by: Roy Golan <rgolan@redhat.com>