By attaching a client to an NBDRequest, we can avoid passing around the
socket descriptor and data buffer.

Also, we can now manage the reference count for the client in
nbd_request_get/put request instead of having to do it ourselved in
nbd_read.  This simplifies things when coroutines are used.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>