VMs can have runtime dependencies - for example it isn't possible to
shutdown netvm used by some other running VM(s). Since client-side tools
may not have full knowledge about rules enforcing those dependencies
(for example may not have access to 'netvm' property), implement
best-effort approach:
1. Try to shutdown all requested VMs
2. For those where shutdown request succeed, wait for actual shutdown
3. For others - go back to step 1
And loop until all VMs are shutdown, or all shutdown requests fails.
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