virtio: Fix warning caused by missing 'static' attribute

Warning from the Sparse static analysis tool:

hw/char/virtio-serial-bus.c:31:3:
 warning: symbol 'vserdevices' was not declared. Should it be static?

Cc: Amit Shah <amit.shah@redhat.com>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/char/virtio-serial-bus.c

@@ -26,7 +26,7 @@
 #include "hw/virtio/virtio-serial.h"
 #include "hw/virtio/virtio-access.h"
 
-struct VirtIOSerialDevices {
+static struct VirtIOSerialDevices {
     QLIST_HEAD(, VirtIOSerial) devices;
 } vserdevices;