frontend: Fix NPE in os info

This patch takes care of console-related methods in AsyncDataProvider.
NPE can occur when existing vm is edited, but its OS was removed from os
info file.

Fixed by handling nulls in AsyncDataProvider and handling non-existent
OS case in client code (UnitVmModel).

Change-Id: I2aef27aee8e079497c9eaa22dfaa6cb9b5502aae
Bug-Url: https://bugzilla.redhat.com/1046809
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>