fix #1734: clone VM: if deactivation fails demote error to warning

fix #1734: clone VM: if deactivation fails demote error to warning

When a template with disks on LVM is cloned to another node, the
volumes are first activated, then cloned and deactivated again after
cloning.

However, if clones of this template are now created in parallel to
other nodes, it can happen that one of the tasks can no longer
deactivate the logical volume because it is still in use.  The reason
for this is that we use a shared lock.
Since the failed deactivation does not necessarily have consequences,
we downgrade the error to a warning, which means that the clone tasks
will continue to be completed successfully.

Signed-off-by: Hannes Duerr <h.duerr@proxmox.com>
Tested-by: Friedrich Weber <f.weber@proxmox.com>