qemu.tests.cpu_hotplug: Call vm.hotplug_vcpu() to hotplug vcpu.

vm.hotplug_vcpu() will select default command for human/qmp monitor.

In some version, cpu_set have been removed from human monitor,
vm.hotplug() will throw TestNAError.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1373 from FengYang/cpu

qemu.tests.cpu_hotplug: Call vm.hotplug_vcpu() to hotplug vcpu.