All incoming messages currently get routed to the generic method
remoteDispatchClientRequest() for processing. To allow incoming
data stream messages to bypass this and be routed to a specific
location, a concept of dispatch filters is introduced.
* qemud/qemud.h: Add a qemud_client_filter struct and a callback
qemud_client_filter_func. Maintain a list of filters on every
struct qemud_client
* qemud/qemud.c: Move remoteDecodeClientMessageHeader() out of
qemudWorker() into qemudDispatchClientRead(). Check registered
message filters in qemudDispatchClientRead() to decide where
to send incoming messages for dispatch.