Slighly improve multi-nic in gnt-instance info

Previously we had only one nic, so the NIC information was all on one
line. This patch changes it to:

    - NICs:
      - nic/0: MAC: aa:00:00:e8:b2:ef, IP: None, bridge: None
      - nic/1: MAC: aa:00:00:53:ca:92, IP: None, bridge: xen-br0
      - nic/2: MAC: aa:00:00:ba:e5:30, IP: None, bridge: None

Reviewed-by: ultrotter