run.py: Support for multiple virtual NICs

This adds a '--nics' command line option to run.py that specifies number
of virtual NICs in the launched VM. For example, to launch QEMU with 4
NICs:

  [penberg@nero osv]$ sudo ./scripts/run.py -n --nics 4
  OSv v0.16-14-g2514672
  eth0: 192.168.122.89
  eth3: 192.168.122.92
  eth2: 192.168.122.91
  eth1: 192.168.122.90

Please note that this only works with DHCP because the command line
interface allows you to only specify MAC address and static IP
configuration for a single NIC.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>