Slightly improve gitian building guide

Change the gitian building guide such that it modifies the VirtualBox NAT
setting (to 192.168.11/24, to make @gavinandresen happy) rather than
overriding Gitian's default IP address in ways that may or may not have
broken/break in the future, considering how fickle/fragile/volatile Gitian
is. Once we're using vboxmanage anyway, replace the multi-step process of
adding the port forwarding rule (and 2 images) with the one-line command
to do the same thing.

Note that this guide may not work in other ways, for other reasons -- for
example, with the new depends system, stuff may need to change. I haven't
gotten around to looking at the changes to gitian building in depth, so I
don't know how much needs to be fixed/rewritten.
[skip ci]