ui: Fix kdump page type initialization

Previously the available kdump target types were initialized during
class creation, this broke doctests and it wouldn't work at runtime,
because the network status can change between page views, and the class
is only instantiated once at application start.
Now the network status is checked on each page view.

Change-Id: I331e27b03e4df5817a318e571a80ad504a0dff4a
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>