tools/xl: Fix exit code for `xl vkbattach`

tools/xl: Fix exit code for `xl vkbattach`

Various version of gcc, when compiling with -Og, complain:

  xl_vkb.c: In function 'main_vkbattach':
  xl_vkb.c:79:12: error: 'rc' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     79 |     return rc;
        |            ^~

The dryrun_only path really does leave rc uninitalised.  Introduce a done
label for success paths to use.

Fixes: a15166af7c3 ("xl: add vkb config parser and CLI")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>