blockcopy: remote implementation for new API
    
    Fairly straightforward - I got lucky that the generated functions
    worked out of the box :)
    
    * src/remote/remote_protocol.x (remote_domain_block_copy_args):
    New struct.
    (REMOTE_PROC_DOMAIN_BLOCK_COPY): New RPC.
    * src/remote/remote_driver.c (remote_driver): Wire it up.
    * src/remote_protocol-structs: Regenerate.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>