core: Fix JSON serialization for diskMap at VmTemplate

JSON de-serialization of java class with a member of an interface type
is loosing its concrete type due to erasure so

 class A {
   private Map x
 }

is serialized to
  A {
    x {}  <- x is considered an arbitraty object
  }

and couldn't deserialized back to Map, throwing an error

while it should have been

 private HashMap x

and then

 A {
   x: [ "java.util.HashMap" , { ... } ]
 }

See Also - http://wiki.fasterxml.com/JacksonPolymorphicDeserialization

Change-Id: Ie56dca3f2cae063c28515e904d644a0f1f39feeb
Signed-off-by: Roy Golan <rgolan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/996005