save last verify result in snapshot manifest

save last verify result in snapshot manifest

Save the state ("ok" or "failed") and the UPID of the respective
verify task. With this we can easily allow to open the relevant task
log and show when the last verify happened.

As we already load the manifest when listing the snapshots, just add
it there directly.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>