block: vdi - use QEMU_PACKED for on-disk structures

The header struct VdiHeader is an on-disk structure for the image
format, and as such should be packed.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vdi.c

@@ -165,7 +165,7 @@ typedef struct {
     uuid_t uuid_link;
     uuid_t uuid_parent;
     uint64_t unused2[7];
-} VdiHeader;
+} QEMU_PACKED VdiHeader;
 
 typedef struct {
     /* The block map entries are little endian (even in memory). */