Add stash_change and stash_pop_change to plugins

Re-add the ability to trigger the validity of fields. Add a
convenience function to drop invalid changes from a field, and
one which triggers invalidity on fields which have not yet had
entry.

Change-Id: I9f82eb54564e8037b74b82817b0e864655400d02
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Let users continue if they cancel the custom device dialog

If users close out of the dialog without saving for some reason,
remove their changes from the plugin model and retrigger
validation so they can continue as normal.

Change-Id: Ic2feb886bbfa738643394ebf540b75f7e84817d9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077786
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Flag the appropriate kdump field if empty

It was possible to attempt to configure kdump with an empty nfs or
ssh entry field after the appropriate checkboxes were ticked. Now,
we should remove the ability to leave it empty

Change-Id: Ib4b879204ac20938c9d760c6d63e0947251238f5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1017083
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>