Commit ffe28ab74b821c916ec4ba8efb5c992454e4bd24 introduced regression
while communicating with older libvirtd command 'domblkstat' used the new
API and did not check for VIR_ERR_RPC error code signalling the remote
server does not support this API and did not fall back to older API.
Thereafter 'domblkstat' ended with "error: unknown procedure: 243".