build: opts-visitor is not really part of QAPI

It is only used by QEMU itself, do not build it into the tests.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qapi/Makefile.objs

 qapi-obj-y = qapi-visit-core.o qapi-dealloc-visitor.o qmp-input-visitor.o
 qapi-obj-y += qmp-output-visitor.o qmp-registry.o qmp-dispatch.o
-qapi-obj-y += string-input-visitor.o string-output-visitor.o opts-visitor.o
+qapi-obj-y += string-input-visitor.o string-output-visitor.o
+
+common-obj-y += opts-visitor.o