<fix>[flat-eip]: reduce EIP migration downtime

[zstack-robot-2]

Backport ZSTAC-87019 EIP migration switchover for ZSTAC-87765.

sync from gitlab !10746

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📥 Commits

Reviewing files that changed from the base of the PR and between 25fd206 and 82225ad.

📒 Files selected for processing (5)

• compute/src/main/java/org/zstack/compute/vm/VmInstanceBase.java
• compute/src/main/java/org/zstack/compute/vm/VmInstanceExtensionPointEmitter.java
• compute/src/main/java/org/zstack/compute/vm/VmMigrateCallExtensionFlow.java
• header/src/main/java/org/zstack/header/vm/VmInstanceMigrateExtensionPoint.java
• plugin/flatNetworkProvider/src/main/java/org/zstack/network/service/flat/FlatEipBackend.java

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