Within the xen-tools log file of the vm i read the following lines:
hook 50-setup-hostname: done.
Running hook 55-create-dev
hook 55-create-dev: done.
Running hook 60-copy-host-files
hook 60-copy-host-files: done.
Running hook 65-copy-user-files
hook 65-copy-user-files: done.
Running hook 70-install-ssh
Generating public/private rsa key pair.
/tmp/j0vB2bPgP1/etc/ssh/ssh_host_rsa_key already exists.
This is exactly as reported here: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=607236
the hook /usr/lib/xen-tools/debian.d/70-install-ssh needlessly generates ssh
keys which stalls xen-create-image.