frontend: Fix NPE in ConsoleProtocol

This patch fixes NPE when changing VM console protocol.

The exception was caused by modifying collection that was created using
Arrays.asList() and this list implementation throws NPE when removing
elements from it.

Fixed by wrapping the list in ArrayList implementation, so that client
code can mutate the collection.

Change-Id: Iaafdc72b83552b51ae65fcf3130b86b1949a3a56
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1109376