vl.c: prohibit simultaneous use of -icount with kvm or xen

With -icount, The vm_clock is updated with help from TCG (it counts
instructions at 2^ICOUNT ns/instructions). With KVM, the instruction
count is not available so KVM cannot provide this help.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vl.c

@@ -3231,6 +3231,11 @@ int main(int argc, char **argv, char **envp)
         fprintf(stderr, "could not initialize alarm timer\n");
         exit(1);
     }
+
+    if (icount_option && (kvm_enabled() || xen_enabled())) {
+        fprintf(stderr, "-icount is not allowed with kvm or xen\n");
+        exit(1);
+    }
     configure_icount(icount_option);
 
     if (net_init_clients() < 0) {