frontend: Fixed NPE on new pool dialog

The 282f87b3b0a96008d87cad21cc48e4d8adcd4cf9 did
bring a regression that the new pool throws an NPE.

The problem was in the fact, that on some places
the osId was taken from the getSelectedItem() and
directly converted to int - but there are cases where
it can be null and that throws an NPE.

Null it can be for example when UnitVmModel.initOSType()
sets getOSType().setItems(AsyncDataProvider.getOsIds()),
which results in ListModel.itemsChanged() which does
setSelectedItems(null); and just after than it sets the
given items.

Change-Id: If6eb8db7e59204a6a77fbd802fcf7a378c31388b
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>