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: NEric Blake <eblake@redhat.com>