dp.h: make format_guid() handle misaligned guid pointers safely.

dp.h: make format_guid() handle misaligned guid pointers safely.

GCC 9 adds -Werror=address-of-packed-member, which causes us to see the
build error reported at
 https://bugzilla.opensuse.org/show_bug.cgi?id=1120862 .

That bug report shows us the following:

In file included from dp.c:26:
dp.h: In function 'format_vendor_helper':
dp.h:120:37: error: taking address of packed member of 'struct <anonymous>' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  120 |  format_guid(buf, size, off, label, &dp->hw_vendor.vendor_guid);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
dp.h:74:25: note: in definition of macro 'format_guid'
   74 |   _rc = efi_guid_to_str(guid, &_guidstr);   \
      |                         ^~~~
cc1: all warnings being treated as errors

This patch makes format_guid() use a local variable as a bounce buffer
in the case that the guid we're passed is aligned as chaotic neutral.

Note that this only fixes this instance and there may be others that bz
didn't show because it exited too soon, and I don't have a gcc 9 build
in front of me right now.

Signed-off-by: Peter Jones <pjones@redhat.com>