qcow2: Explicit mention of padding bytes

Although we already covered the need for padding bytes with our
changes in commit 3ae3fcf, commit 66fcbca (both v5.0.0) added one
byte and relied on the rest of the text for implicitly covering 7
padding bytes.  For consistency with other parts of the header (such
as the header extension format listing padding from n - m, or the
snapshot table entry listing variable padding), we might as well call
out the remaining 7 bytes as padding until such time (as any) as they
gain another meaning.

Signed-off-by: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230522184631.47211-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

docs/interop/qcow2.txt

@@ -226,6 +226,7 @@ version 2.
                     <https://www.zlib.net/> in QEMU. However, clusters with the
                     deflate compression type do not have zlib headers.
 
+        105 - 111:  Padding, contents defined below.
 
 === Header padding ===