This removes the section on packages from installing.sgml (until we have
packages) and adds/corrects informations about using the startup script.

It also adds notes about the ``--master-netdev`` option, which one needs
to use if the node doesn't have an interface named xen-br0 (or if it's
not the interface wanted for external access).

This changes installing.sgml, adding details about required software and
OS instances. It also updates INSTALL and README to correspond with the
sgml manual.

Reviewed-by: ultrotter

Reviewed-by: iustinp

Reviewed-by: iustinp