sync_disks: use allow_rename to avoid collisions on the target storage

sync_disks: use allow_rename to avoid collisions on the target storage

This makes it possible to migrate a VM with volumes store1:vm-123-disk-0
store2:vm-123-disk-0 to some targetstorage. Also prevents migration failure
when there is an orphaned disk with the same volid on the target.

To avoid confusion, the name should not change for 'vmstate'-volumes.

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