qcow2: Alignment of snapshot table entries

The qcow2 specification does not explicitly state so far that every
snapshot table entry is aligned to 8 bytes. QEMU, in contrast, does this
alignment, thus it should be properly documented (which this patch
does).
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/specs/qcow2.txt

@@ -355,3 +355,6 @@ Snapshot table entry:
         variable:   Unique ID string for the snapshot (not null terminated)
 
         variable:   Name of the snapshot (not null terminated)
+
+        variable:   Padding to round up the snapshot table entry size to the
+                    next multiple of 8.