plugin: hooks: add explicit returns

to avoid returning something unexpected. Finish what
afeda18256 already started for all the other
plugins. At least for ZFS's on_add_hook this is necessary (adding a ZFS storage
currently fails as reported here [0]), but it cannot hurt
in the other places either as the only hooks we expect to return something
currently are PBS's on_add_hook and on_update_hook.

[0]: https://forum.proxmox.com/threads/gui-add-zfs-storage-verification-failed-400-config-type-check-object-failed.79734/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

PVE/Storage/LVMPlugin.pm

@@ -269,6 +269,8 @@ sub on_add_hook {
 
 	lvm_create_volume_group($path, $scfg->{vgname}, $scfg->{shared});
     }
+
+    return;
 }
 
 sub parse_volname {