w32: Disable buffering for log file

W32 does not support line buffering, but it supports unbuffered output.

Unbuffered output is better for writing to qemu.log than fully buffered
output because it also shows the latest log messages when an application
crash occurs.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
