libqemustub: sysbus_get_default() stub

The stub will be used on cases where sysbus.c is not compiled in (e.g.
*-user).

Note that code that uses NULL as the bus with qdev{_try,}_create()
implicitly uses sysbus_get_default() as the bus, and will still require
sysbus.c to be compiled in.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

stubs/Makefile.objs

@@ -7,4 +7,5 @@ stub-obj-y += get-fd.o
 stub-obj-y += set-fd-handler.o
 stub-obj-y += reset.o
 stub-obj-y += vmstate.o
+stub-obj-y += sysbus.o
 stub-obj-$(CONFIG_WIN32) += fd-register.o

stubs/sysbus.c

+#include "hw/qdev-core.h"
+
+BusState *sysbus_get_default(void)
+{
+    return NULL;
+}