UI: Prevent dialog close when pressing Enter in TextArea

By default add/edit VM dialog close when pressing Enter (for fast
navigation), This is a problem while edition TextArea when the enter
key is used.

Bug-Url: https://bugzilla.redhat.com/1051041
Change-Id: I436d4ab656d2739a9681a6b3d2f80038eb0c2063
Signed-off-by: Shahar Havivi <shaharh@redhat.com>