qcow2: Write v3-compliant snapshot list on upgrade

qcow2: Write v3-compliant snapshot list on upgrade

qcow2 v3 requires every snapshot table entry to have two extra data
fields: The 64-bit VM state size, and the virtual disk size.  Both are
optional for v2 images, so they may not be present.

qcow2_upgrade() therefore should update the snapshot table to ensure all
entries have these extra data fields.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1727347
Reported-by: Eric Blake <[email protected]>
Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 20191011152814[email protected]
Signed-off-by: Max Reitz <[email protected]>