When some VM timeout on shutdown, the tool will try to kill all of them,
but at this point some of them may be already powered off (not all
hanged during shutdown, but only some). Handle this
situation instead of crashing. And add appropriate test.
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.
