prep-vagrant-disk: Print when we're creating files

For easier debugging.
prep*disk: Use tmpdir for mount point, not /mnt
guestfish: Use verbose unmount

For easier debugging.
prep*disk: Do SELinux relabeling

Otherwise it doesn't really work in enforcing mode.
guestfish: Synchronously watch pid file for completion

guestunmount is actually asynchronous, we still need to watch the pid
file.  However, we can do this better than the old polling loop via
inotify.
prep*disk: Fix SELinux relabeling

We need to relabel relative to the deployment root, so make use of new
OSTree functionality for that.
packaging: Uncache Dockerfile again