pc: make sure that plugged CPUs are of the same type

heterogeneous cpus are not supported and hotplugging different
cpu model crashes QEMU:

  qemu-system-x86_64 -cpu qemu64 -smp 1,maxcpus=2
  (qemu) device_add host-x86_64-cpu,socket-id=1,core-id=0,thread-id=0,id=foo
  (qemu) info cpus
  error: failed to get MSR 0x38d
  qemu-system-x86_64: target/i386/kvm.c:2121: kvm_get_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
  Aborted (core dumped)

Gracefully fail hotplug process in case of user mistake.
Reported-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Message-Id: <1507638879-200718-1-git-send-email-imammedo@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/i386/pc.c

@@ -1876,8 +1876,15 @@ static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
     CPUArchId *cpu_slot;
     X86CPUTopoInfo topo;
     X86CPU *cpu = X86_CPU(dev);
+    MachineState *ms = MACHINE(hotplug_dev);
     PCMachineState *pcms = PC_MACHINE(hotplug_dev);
 
+    if(!object_dynamic_cast(OBJECT(cpu), ms->cpu_type)) {
+        error_setg(errp, "Invalid CPU type, expected cpu type: '%s'",
+                   ms->cpu_type);
+        return;
+    }
+
     /* if APIC ID is not set, set it based on socket/core/thread properties */
     if (cpu->apic_id == UNASSIGNED_APIC_ID) {
         int max_socket = (max_cpus - 1) / smp_threads / smp_cores;