kvm: add error message for when SMP is requested

Right now, if you try e.g. '-smp 2' you just get 'failed to
initialize KVM'.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

kvm-all.c

@@ -338,8 +338,10 @@ int kvm_init(int smp_cpus)
     int ret;
     int i;
 
-    if (smp_cpus > 1)
+    if (smp_cpus > 1) {
+        fprintf(stderr, "No SMP KVM support, use '-smp 1'\n");
         return -EINVAL;
+    }
 
     s = qemu_mallocz(sizeof(KVMState));