core: Changed QPID port type from int to Integer

Previous implementation was NPE-prone due to the getter/setter
relating to the field as Integer, while it was actually defined as
int.

Change-Id: Iaadefc98dcd34a7158f1d93fa3ca5f60a26728f3
Signed-off-by: Lior Vernia <lvernia@redhat.com>