Avoid unused variable warnings:
qemu-char.c: In function 'qmp_chardev_open_port':
qemu-char.c:3132: warning: unused variable 'fd'
qemu-char.c:3132: warning: unused variable 'flags'

in configurations with neither HAVE_CHARDEV_TTY nor
HAVE_CHARDEV_PARPORT set.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>