lwt: fix a race in the rpc function

We must add the request id to the table before transmitting, in
case the reply comes back very quickly.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
lwt: treat an unexpected packet as an error

If an unexpected packet comes back then we shouldn't assume the
data is intact.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>