I can use this command to create a working VM running the "precise" (12.04 LTS) Ubuntu release:

vmbuilder kvm ubuntu -v --suite=precise \
--libvirt=qemu:///system --arch=amd64 \
--cpus=2 --mem=2047 --swapsize=4096 --rootsize=71680 --flavour=server \
--hostname=xxx --mirror=http://archive.ubuntu.com/ubuntu \
--components=main,universe --addpkg=openssh-server --user=claus \
--pass=xxxx --dest=/home/claus/vm-xxx --bridge=br0

This appears to work fine.

But if I replace "precise" with "trusty" (14.04 LTS), the build process halts after a few minutes.

In the log file from the unsuccessful "trusty" build, the last lines are:

2015-05-16 12:03 DEBUG : I: Configuring ubuntu-minimal...
2015-05-16 12:03 DEBUG : I: Configuring libc-bin...
2015-05-16 12:03 DEBUG : I: Configuring initramfs-tools...
2015-05-16 12:03 DEBUG : I: Base system installed successfully.
At this point, nothing further happens. The vmbuilder program hasn't terminated, but nothing is happening.

In the log file from the successful "precise" build, the above lines are followed by

2015-05-16 11:41 INFO : Calling hook: configure_os
and the build process continues.

(I am running the commands on a "precise" (12.04) system.)

What am I doing wrong in the "trusty" build?