Migrate vm across clusters

[ustcweizhou]

Description

This PR adds a global setting migrate.vm.across.clusters to indicate whether vm can be live migrated to other clusters.

If vm is running on the last host in a cluster and vm cannot be migrated to other clusters, put host to maintenance will fail.

This is based on pr #4378

This fixes #3707 #3720

Some rules
(1) for vmware, across-cluster migration of vms with cluster-scoped pools is supported
(2) for other hypervisors except vmware, vm can be live migrated to other clusters (with same hypervisor type), if all volumes are in zone-wide storage pool.
(3) migration of systemvms (CPVM, SSVM) is only possible across clusters in same pod to avoid potential network issues.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Screenshots (if appropriate):

How Has This Been Tested?