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 <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20191011152814.14791-5-mreitz@redhat.com
[mreitz: Adjusted comments as proposed by Eric]
Signed-off-by: Max Reitz <mreitz@redhat.com>