gdbstub: revert to previous set_reg behaviour

gdbstub: revert to previous set_reg behaviour

The refactoring of handle_set_reg missed the fact we previously had
responded with an empty packet when we were not using XML based
protocols. This broke the fallback behaviour for architectures that
don't have registers defined in QEMU's gdb-xml directory.

Revert to the previous behaviour and clean up the commentary for what
is going on.

Fixes: 62b3320bddd
Signed-off-by: Alex Bennée <[email protected]>
Tested-by: Mark Cave-Ayland <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Cc: Jon Doron <[email protected]>