webadmin: Implement generic sorting for Identifiable columns

Taking advantage of Identifiable's getValue() method, implemented a
generic comparator to handle classes implementing the interface. Also
applied it to the status columns in the Network/Hosts and Network/VMs
subtabs.

Change-Id: Ia1433e6650f53229bf5e02439b5d26dfb56014c0
Signed-off-by: Lior Vernia <lvernia@redhat.com>