Use debian boot cdrom for initvm initialisation.

The initial virtual machine (initvm) was initialised using the elbe-bootstrap.deb package.
Architectures that were not in the debian package (e.g. i386) could not be used.
Now the initvm is initialised from a debian boot cdrom. So alot more architectures should be
available.

The cdrom to use for that can be configured by /initvm/installer/cdrom.iso. Also the path
to extract a kernel (vmlinux) and an initial ram disk (initrd.gz) from the cdrom can be
configured by /initvm/installer/vmlinux and /initvm/installer/initrd.gz.
Without any configuration a default netinst cdrom from the choosen debian suite is used.
Remove left over from earlier tests. Use correct xml key.
Use correct debian architecture for x86_64 machine.
Assure aptitude is available for 2nd stage boot.
Incomplete changes to use iso installer images for initvm, buildimage and target.
init-vm: forward host hw random generator into vm.

From devel/elbe-1.0. Adapted to 2stage init process.
Improve elbe init from cdrom iso image.

Deduce default iso image from suite and architecture.
Deduce architecture if not explicitly given.