rpc: Double buffer size instead of quadrupling buffer size.
    
    When increasing the buffer size up to VIR_NET_MESSAGE_MAX, we
    currently quadruple it each time.  This unfortunately means that we
    cannot allow certain buffer sizes -- for example the current
    VIR_NET_MESSAGE_MAX == 33554432 can never be "hit" since ‘newlen’
    jumps from 16MB to 64MB.
    
    Instead of quadrupling, double it each time.
    
    Thanks: Daniel Berrange.
    Signed-off-by: Richard W.M. Jones <rjones@redhat.com>