qemu: agent: Make setting of vcpus more robust

qemu: agent: Make setting of vcpus more robust

Documentation for the "guest-set-vcpus" command describes a proper
algorithm how to set vcpus. This patch makes the following changes:

- state of cpus that has not changed is not updated
- if the command was partially successful the command is re-tried with
  the rest of the arguments to get a proper error message
- code is more robust against malicious guest agent
- fix testsuite to the new semantics