linux-user/main.c: Add option to user-mode emulation so that user can specify log...

linux-user/main.c: Add option to user-mode emulation so that user can specify log file name

  QEMU linux user-mode's default log file name is "/tmp/qemu.log". In order to
change the log file name, user need to modify the source code then recompile
QEMU. This patch allow user use "-D logfile" option to specify the log file
name.

Signed-off-by: Chen Wen-Ren <chenwj@iis.sinica.edu.tw>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>