Move singlestep_enabled to CPUState.

The previous patch forgot to move this field from CPU_COMMON to
CPUState, so do it here.

Change-Id: I71ed4605c939eb6c23d3e70b08ea89e59842cdd0
diff --git a/kvm-all.c b/kvm-all.c
index bf72708..4edab48 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -914,7 +914,7 @@
     struct kvm_guest_debug dbg;
 
     dbg.control = 0;
-    if (env->singlestep_enabled)
+    if (ENV_GET_CPU(env)->singlestep_enabled)
         dbg.control = KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP;
 
     kvm_arch_update_guest_debug(env, &dbg);