Unregister event callback if a fatal error occurs during dispatch

Unregister event callback if a fatal error occurs during dispatch

If we get an I/O error in the async event callback for an RPC
client, we might not have consumed all pending data off the
wire. This could result in the callback being immediately
invoked again. At which point the same I/O might occur. And
we're invoked again. And again...And again...

Unregistering the async event callback if an error occurs is
a good safety net. The real error will be seen when the next
RPC method is invoked

* src/rpc/virnetclient.c: Unregister event callback on error