[pve-devel] fix #503 - print correct offline time for VM stop mode backups

[Thomas Lamprecht]

Encode the time a guest was stopped in the backup $task hash, also allow a
plugin to set the time the guest was continued (is running again) from inside
the plugin, which then allows to fix #503, i.e., stop mode backups for VM where
we stopped the VM, started the VM again in backup mode and then sent a "cont"
to it, which made it available after a few seconds, but the log still told an
user that it was only online again after the backup was fully finished.