core: VmBase - remove final keyword from diskList

Removing 'final' keyword from diskList member in VmBase.
GWT RPC doesn't serialize 'private final' members
(i.e. diskList is returned as null to the UI).

Change-Id: I372b3b2ba5abcbc2720fa0d3b7d20eabb0750241
Signed-off-by: Daniel Erez <derez@redhat.com>