Use the oper_ram field if available

For the RAPI and LUXI backends, we can get the actual memory usage (if
instances are running) via the oper_ram, whereas backend/memory only
tell what the instance will use at the next boot.

Not using oper_ram means that the node model is flawed and we consider
wrong values for the instance's memory (resulting sometimes in hilarious
values such as x_mem = -700 MB).