vos: preserve cloneId and backupId when restoring

vos: preserve cloneId and backupId when restoring

Preserve the volume clone and backup ids in the volume header when
restoring over an existing volume, instead of always setting the clone
and backup ids to zero.

For example, before this change, restoring over a volume resets the
ROnly and Backup ids reported in the volume header section of vos
examine.

    $ vos examine xyzzy
    xyzzy                          536871023 RW          3 K  On-line
        myhost /vicepa
        RWrite  536871023 ROnly  536871024 Backup  536871025
     ...
     RWrite: 536871023     ROnly: 536871024     Backup: 536871025
        number of sites -> 2
           server myhost partition /vicepa RW Site
           server myhost partition /vicepa RO Site

    $ cat /tmp/xyzzy.dump | vos restore myhost a xyzzy -overwrite incremental
    Restoring volume xyzzy Id 536871023 on server myhost partition /vicepa .. done
    Restored volume xyzzy on myhost /vicepa

    $ vos examine xyzzy
    xyzzy                          536871023 RW          3 K  On-line
        myhost /vicepa
        RWrite  536871023 ROnly          0 Backup          0
     ...
     RWrite: 536871023     ROnly: 536871024     Backup: 536871025
        number of sites -> 2
           server myhost partition /vicepa RW Site
           server myhost partition /vicepa RO Site

Reviewed-on: http://gerrit.openafs.org/11516
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
(cherry picked from commit 7266685a03b12ca03c247623e7ffdc054a017382)

Change-Id: Iad0ef8f2d3fa1ef87b87bcbf9f74735ede1c2140
Reviewed-on: http://gerrit.openafs.org/12082
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>