cpu: Move singlestep_enabled field from CPU_COMMON to CPUState

Prepares for changing cpu_single_step() argument to CPUState.

Acked-by: Michael Walle <michael@walle.cc> (for lm32)
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/kvm-all.c b/kvm-all.c
index 232c39a..a210389 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1890,7 +1890,7 @@
 
     data.dbg.control = reinject_trap;
 
-    if (env->singlestep_enabled) {
+    if (cpu->singlestep_enabled) {
         data.dbg.control |= KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP;
     }
     kvm_arch_update_guest_debug(cpu, &data.dbg);