kvmclock: Don't crash QEMU if KVM is disabled

Most machines don't allow sysbus devices like "kvmclock" to be
created from the command-line, but some of them do (the ones with
has_dynamic_sysbus=true). In those cases, it's possible to
manually create a kvmclock device without KVM being enabled,
making QEMU crash:

  $ qemu-system-x86_64 -machine q35,accel=tcg -device kvmclock
  Segmentation fault (core dumped)

This changes kvmclock's realize method to return an error if KVM
is disabled, to ensure it won't crash QEMU.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170309185046.17555-1-ehabkost@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/i386/kvm/clock.c

@@ -22,6 +22,7 @@
 #include "kvm_i386.h"
 #include "hw/sysbus.h"
 #include "hw/kvm/clock.h"
+#include "qapi/error.h"
 
 #include <linux/kvm.h>
 #include <linux/kvm_para.h>
@@ -208,6 +209,11 @@ static void kvmclock_realize(DeviceState *dev, Error **errp)
 {
     KVMClockState *s = KVM_CLOCK(dev);
 
+    if (!kvm_enabled()) {
+        error_setg(errp, "kvmclock device requires KVM");
+        return;
+    }
+
     kvm_update_clock(s);
 
     qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);