vl.c: Don't zero-initialize statics for serial_hds

vl.c: Don't zero-initialize statics for serial_hds

checkpatch reminds us that statics shouldn't be zero-initialized:

ERROR: do not initialise statics to 0 or NULL
#35: FILE: vl.c:157:
+static int num_serial_hds = 0;

ERROR: do not initialise statics to 0 or NULL
#36: FILE: vl.c:158:
+static Chardev **serial_hds = NULL;

I forgot to fix this in 6af2692e86f9fdfb3d; do so now.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180426140253.3918-1-peter.maydell@linaro.org