Define an API for registering incoming message dispatch filters

Define an API for registering incoming message dispatch filters

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.