Description of problem:

If host arch != guest arch, the virt-builder --selinux-relabel
flag fails:

$ virt-builder --arch ppc64 fedora-21 --selinux-relabel
[   1.0] Downloading: http://libguestfs.org/download/builder/fedora-21-ppc64.xz
[   1.0] Planning how to build this image
[   1.0] Uncompressing
[  14.0] Opening the new disk
[  27.0] Setting a random seed
[  27.0] Setting passwords
virt-builder: Setting random password of root to 9XFmS9JoU10hYuWf
[  28.0] SELinux relabelling
virt-builder: warning: log file /tmp/builder.log: Guestfs.Error("download: 
/tmp/builder.log: No such file or directory") (ignored)
virt-builder: error: load_policy && fixfiles restore: command exited with 
an error

This is because the code tries to run binaries from the
guest (load_policy etc), but it should skip that step and just
touch /.autorelabel directly.

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

libguestfs-1.29.36-1.fc23.x86_64

How reproducible:

100%

Steps to Reproduce:
1. See command above.