If the memory of guest OS is changed constantly, the live migration
can not be ended ever for ever.

We can use the command 'virsh migrate-setmaxdowntime' to control the
live migration. But the value of maxdowntime is diffcult to calculate
because it depends on the transfer speed of network and constantly
changing memroy size. We need a easy way to control the live migration.

This patch adds the support of forcing guest to suspend at timeout.
With this patch, when we migrate the guest OS, we can specify a
timeout. If the live migration timeouts, auto-suspend the guest OS,
where the migration will complete offline.