virtchnl: fix m68k build.

The kernel test robot reported a build failure on m68k in the intel
driver due to the recent shapers-related changes.

The mentioned arch has funny alignment properties, let's be explicit
about the binary layout expectation introducing a padding field.

Fixes: 608a5c05c39b ("virtchnl: support queue rate limit and quanta size configuration")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410131710.71Wt6LKO-lkp@intel.com/
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/e45d1c9f17356d431b03b419f60b8b763d2ff768.1729000481.git.pabeni@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>