rpc: Double buffer size instead of quadrupling buffer size.

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>