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>

gdbstub/internals.h

@@ -33,6 +33,7 @@ typedef struct GDBProcess {
     uint32_t pid;
     bool attached;
 
+    /* If gdb sends qXfer:features:read:target.xml this will be populated */
     char *target_xml;
 } GDBProcess;