fuzz: add an "opaque" to the FuzzTarget struct

It can be useful to register FuzzTargets that have nearly-identical
initialization handlers (e.g. for using the same fuzzing code, with
different configuration options). Add an opaque pointer to the
FuzzTarget struct, so that FuzzTargets can hold some data, useful for
storing target-specific configuration options, that can be read by the
get_init_cmdline function.

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20201023150746.107063-14-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/fuzz/fuzz.h

@@ -100,6 +100,7 @@ typedef struct FuzzTarget {
                        uint8_t *out, size_t max_out_size,
                        unsigned int seed);
 
+    void *opaque;
 } FuzzTarget;
 
 void flush_events(QTestState *);