crypto/block: remove redundant struct packing to fix build with gcc 9

crypto/block: remove redundant struct packing to fix build with gcc 9

Build fails with gcc 9:

crypto/block-luks.c:689:18: error: taking address of packed member of ‘struct QCryptoBlockLUKSHeader’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
  689 |     be32_to_cpus(&luks->header.payload_offset);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
crypto/block-luks.c:690:18: error: taking address of packed member of ‘struct QCryptoBlockLUKSHeader’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
  690 |     be32_to_cpus(&luks->header.key_bytes);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~
crypto/block-luks.c:691:18: error: taking address of packed member of ‘struct QCryptoBlockLUKSHeader’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
  691 |     be32_to_cpus(&luks->header.master_key_iterations);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

... a bunch of similar errors...

crypto/block-luks.c:1288:22: error: taking address of packed member of ‘struct QCryptoBlockLUKSKeySlot’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
 1288 |         be32_to_cpus(&luks->header.key_slots[i].stripes);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

All members of the QCryptoBlockLUKSKeySlot and QCryptoBlockLUKSHeader are
naturally aligned and we already check at build time there isn't any
unwanted padding. Drop the QEMU_PACKED attribute.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>