iso-dialog: Avoid crash when closing dialog early
    
    We must take into account that users can close the dialog at anytime,
    even during an operation of fetch or set ISO has not been finished. This
    will cause the callbacks for those operations to be invoked with an
    invalid object, crashing the application.
    
    To fix this issue we need to pass a GCancellable to the asynchronous
    operations, so they can be cancelled if the dialog happens to get closed
    before they complete.
    
    NOTE: This patch triggers a deadlock in libgovirt when the dialog is
    closed before the operation completes. Bug reported in
    https://bugzilla.gnome.org/show_bug.cgi?id=777808. We will need to bump
    libgovirt dependency whenever it has a new release.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>