Fix memory leaks in libvirtd's message processing

Fix memory leaks in libvirtd's message processing

Commit 47cab734995fa9521b1df05d37e9978eedd8d3a2 changed the way how
qemud_client_message objects were reused. Before this commit
remoteDispatchClientRequest() reused the received message for normal responses
and to report non-fatal errors. If a fatal error occurred qemudWorker() frees
the message. After this commit non-fatal errors are reported by
remoteSerializeReplyError() using a new qemud_client_message object and the
original message leaks.

To fix this leak the original message has to be freed if
remoteSerializeReplyError() succeeds. If remoteSerializeReplyError()
fails the original message is freed in qemudWorker().

* daemon/dispatch.c: free qemud_client_message objects that will not be reused
  and would leak otherwise, also free the allocated qemud_client_message object
  in remoteSerializeError() if an error occurs