block: qed - use QEMU_PACKED for on-disk structures

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>

block/qed.h

@@ -100,7 +100,7 @@ typedef struct {
     /* if (features & QED_F_BACKING_FILE) */
     uint32_t backing_filename_offset; /* in bytes from start of header */
     uint32_t backing_filename_size;   /* in bytes */
-} QEDHeader;
+} QEMU_PACKED QEDHeader;
 
 typedef struct {
     uint64_t offsets[0];            /* in bytes */