qcow2: Refuse to get unaligned offsets from cache

Instead of using an assertion, it is better to emit a corruption event
here.  Checking all offsets for correct alignment can be tedious and it
is easily possible to forget to do so.  qcow2_cache_do_get() is a
function every L2 and refblock access has to go through, so this is a
good central point to add such a check.

And for good measure, let us also add an assertion that the offset is
non-zero.  Making this a corruption event is not feasible, because a
zero offset usually means something special (such as the cluster is
unused), so all callers should be checking this anyway.  If they do not,
it is their fault, hence the assertion here.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171110203111.7666-6-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
