Add support for multiple networked qemu instances

When invoked with an index parameter, qemu.sh will launch a qemu
instance prepared to run concurrently and network with other qemu
instances on the same host.  So you should now be able to do

 board/qemu/qemu.sh 0
 board/qemu/qemu.sh 1
 board/qemu/qemu.sh 3
...

on different shells and get separate qemu instances that can ping each
other.

For each instance, qemu.sh will now:

1. Create a clone rootfs image
2. Create a separate tap interface
3. Set a different MAC address (-device e1000,...,mac=....)
4. Pick a new port for qemu's gdb server (-gdb tcp:...)

In the first instantiation of an instance you will need to change its IP
address by editing /etc/distro11s.conf.  The change is persistent, and
will only need to be done again when you delete the instance's
out/qemu/rootfs-<index>.cow2 file.

Signed-off-by: Javier Cardona <javier@cozybit.com>