Improve xl socat migrations

Improve xl socat migrations

Improve their reliability by allowing socat to reuse an address,
preventing errors if migrations are executed with too narrow a
timespan.

Improve performance by choosing a better blocksize than the default 8k.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>