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: Greg Kurz <groug@kaod.org>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1507638879-200718-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>