fix #2512: post-stop: unmount stage mps before cleanup

fix #2512: post-stop: unmount stage mps before cleanup

With staged mount points we now have mount points also
mounted in our staging temp directory, and we keep them
there in order to prevent hotplugged mounts (which can be
unmounted by the container) to disconnect from their loop
devices, so we need to clean those up as well before we can
run any cleanups.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>