tools/libxl: Fix uninitialised variable in libxl__write_stub_dmargs()

tools/libxl: Fix uninitialised variable in libxl__write_stub_dmargs()

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

  libxl_dm.c: In function ‘libxl__write_stub_dmargs’:
  libxl_dm.c:2166:16: error: ‘dmargs’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
               rc = libxl__xs_write_checked(gc, t, path, dmargs);
               ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It isn't actually used while uninitialised, but only because of how the
is_linux_stubdom checks line up.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>