QEDHeader is read, and written, directly from on-disk images
via bdrv_pread()/write().  To avoid any unintentional padding,
these structs should be packed.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>