Add disk copy support at backend and the rpc level

This uses a simple 'dd if=… | ssh $target dd of=…' method, like the
ExportSnapshot (which uses the OS export; here we want full disk-level
copy and not any FS-level changes).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>