gdbstub: replace global gdb_has_xml with a function

Try and make the self reported global hack a little less hackish by
providing a query function instead. As gdb_has_xml was always set if
we negotiated XML we can now use the presence of ->target_xml as the
test instead.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-12-alex.bennee@linaro.org>

include/exec/gdbstub.h

@@ -31,12 +31,12 @@ int gdbserver_start(const char *port_or_device);
 void gdb_set_stop_cpu(CPUState *cpu);
 
 /**
- * gdb_has_xml:
- * This is an ugly hack to cope with both new and old gdb.
- * If gdb sends qXfer:features:read then assume we're talking to a newish
- * gdb that understands target descriptions.
+ * gdb_has_xml() - report of gdb supports modern target descriptions
+ *
+ * This will report true if the gdb negotiated qXfer:features:read
+ * target descriptions.
  */
-extern bool gdb_has_xml;
+bool gdb_has_xml(void);
 
 /* in gdbstub-xml.c, generated by scripts/feature_to_c.sh */
 extern const char *const xml_builtin[][2];