rpc: Don't rewrite msg->fds on every read dispatch

rpc: Don't rewrite msg->fds on every read dispatch

When we are receiving data in smaller chunks it might happen that
virNetServerClientDispatchRead() will be called multiple times.  And as
that happens, if it is a message that also transfer headers, we decode
the number of them every single time and, unfortunately, also allocate
the memory for them.  That causes a leak, in the best scenario.

Best viewed with '-w'.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>