nitvm: Little cleaning in the code of the vm

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Implementing attribute access with direct access

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Attributes access is fully functionnal with direct access when possible

Signed-off-by: Julien Pagès <julien.projet@gmail.com>