Thanks for all the comments!

This is what I do now:

In domU
-------
su -
dd if=/dev/zero of=/var/tmp/zero bs=1m &
rm -f /var/tmp/zero
fg
halt -p

In dom0
-------
gzip -9 p133

-rw-r--r--  1 root  wheel   10G Jan 25 15:15 p133
-rw-r--r--  1 root  wheel  193M Jan 25 15:15 p133.gz