Description of problem:

  $ virt-sysprep -a debian-9
  ...
  virt-sysprep: warning: random seed could not be set for this type of guest

This is a newly created Debian 9 guest, so d-i has run but the
guest has never been booted normally.

Debian 9 uses systemd and systemd-random-seed.service.  The
service file contains:

  [Unit]
  ...
  RequiresMountsFor=/var/lib/systemd/random-seed

which seems to indicate that this is the path where the
random seed would be stored.

However this path does not exist, and virt-customize works
by assuming that the seed file exists before it will detect
and overwrite the seed.  I'm guessing this file would be
created at first boot of the guest, which is too late.

Version-Release number of selected component (if applicable):

libguestfs-tools-c-1.37.14-3.fc27.x86_64

How reproducible:

At least once.

Steps to Reproduce:
1. Create a new guest using d-i.
2. Run virt-sysprep on it.