rpc: fix buffer offset updates after decoding payload

rpc: fix buffer offset updates after decoding payload

The buffer length refers to the allocated buffer memory size,
while the offset refers to have much of the buffer we have
read/written. After reading the message payload we must thus
update the latter.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>