fs_utils: fix new (un)persist logging statements

The new persistence code added several logging entries using string
formatting instead of logger formatting. On top of that, three
entries were missing the value.

Change-Id: I075d58500760cfdea683731838e53c13fcd8290c
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
utils_fs: fix _add_path_entry not to add duplicates

_del_path_entry correctly strips the entries in /config/files to do
the matching against the path to persist. However, _add_path_entry
was missing this step, resulting in never matching. This patch
addresses the issue.

Change-Id: Ic3806ef126e1445853938c383d16cc116c553309
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
utils_fs: cleanup empty dirs on unpersist

Up until now, when unpersisting a path like:

    /config/var/run/vdsm/persistence/foo

in /config there would be remains stored forever:

    /config/var/run/vdsm/persistence

With this patch, the parent directory of the persisted path will be
iteratively cleaned upwards so that empty directories are pruned
from /config

Change-Id: I0431974804819c5d4f587165cae7be665529b345
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>