util/log: Remove qemu_log_flush

All uses flush output immediately before or after qemu_log_unlock.
Instead of a separate call, move the flush into qemu_log_unlock.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-20-richard.henderson@linaro.org>