Define the IDL for io.systemd.MachineInstance.ReplaceStorage, a
runtime hot-swap of an already-attached storage volume's backing
file. The signature mirrors AddStorage minus the 'config' field
because the device frontend (virtio-blk, virtio-scsi, nvme, scsi-cd)
does not change — only the backing file behind it.
The implementation lives in vmspawn (next commit) and uses QMP
blockdev-reopen to swap the file child of the existing format node.
The reused error vocabulary (NoSuchStorage, StorageImmutable,
NotConnected, plus the generic errno path) covers every failure
mode; no new errors are added.
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>