Description of problem:

There should be a virt-customize --truncate option which
truncates a file (keeping its permissions etc).  The
use for this is to truncate log files and so on, eg.

 virt-builder --truncate /var/log/messages

Maybe even consider a recursive truncate which truncates
all files in a directory, eg:

 virt-builder --truncate-recursive /var/log

Additional information:

Suggested by NuxRo on IRC.