The format was dropped in QEMU binary version 2.2 with commit
550830f935 ("block: delete cow block driver").
This follows qemu-server commit "drive: remove ancient 'cow' from
formats".
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Acked-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>