Refactor the code flow so that 'exit_monitor:' can be removed.
This patch also moves the auditing and setting of the new vCPU count
right to the place where the hotplug happens, since it's possible that
the hotplug succeeds and adds a cpu while other stuff fails.
Lastly, failures of qemuMonitorGetCPUInfo are now reported rather than
ignored. The function retuns 0 if it "successfully" detected 0 threads.