block-coroutine-wrapper: Allow arbitrary parameter names

Don't assume specific parameter names like 'bs' or 'blk' in the
generated code, but use the actual name.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230911094620.45040-8-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>