qcow2: Keep unknown extra snapshot data

qcow2: Keep unknown extra snapshot data

The qcow2 specification says to ignore unknown extra data fields in
snapshot table entries.  Currently, we discard it whenever we update the
image, which is a bit different from "ignore".

This patch makes the qcow2 driver keep all unknown extra data fields
when updating an image's snapshot table.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 20191011152814[email protected]
[mreitz: Adjusted comments as proposed by Eric]
Signed-off-by: Max Reitz <[email protected]>