Extract the call to qemuDomainSelectHotplugVcpuEntities outside of
qemuDomainSetVcpusLive and decide whether to hotplug or unplug the
entities specified by the cpumap using a boolean flag.

This will allow to use qemuDomainSetVcpusLive in cases where we prepare
the list of vcpus to enable or disable by other means.