sw64: kvm: fix clock synchronization of VM live migration

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73KSP



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When a VM is migrated more than once, the current clock synchronization
mechanism causes the correct longtime of source VM to be lost, so it
should get the compensated longtime value when the live migration
precopy is compelted for subsequent clock synchronization.

Signed-off-by: Min Fanlei <minfanlei@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>