page-xfer: make err-message reflect what realy happen

Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
zdtm: add --track-mem to final snapshot

otherwize it won't use/check parent snapshots, only last one
on restore.

Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>