restapi: Fix findbug error - wrong key type

The key type should be a Guid instead of the String
which is passed to query map of type
Map<Guid, VdsNetworkInterface>.

Change-Id: If29de8662dfcdf58f063e09083d241f91e4a8ebf
Signed-off-by: Moti Asayag <masayag@redhat.com>