rbd: unmap volume after rename

rbd: unmap volume after rename

When krbd is used, subsequent removal after an an operation
involving a rename could fail with
> librbd::image::PreRemoveRequest: 0x559b7506a470 \
> check_image_watchers: image has watchers - not removing
because the old mapping was still present.

For both operations with a rename, the owning guest should be offline,
but even if it weren't, unmap simply fails when the volume is in-use.

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