kvm-all: make async_safe_run_on_cpu safe on kvm too

Wrap the bulk of kvm_cpu_exec with cpu_exec_start/end, so that kvm
version can also enjoy performing certain operations while all vCPUs are
quiescent.
Signed-off-by: NRoman Kagan <rkagan@virtuozzo.com>
Message-Id: <20170606181948.16238-15-rkagan@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

kvm-all.c

@@ -1977,6 +1977,7 @@ int kvm_cpu_exec(CPUState *cpu)
     }
 
     qemu_mutex_unlock_iothread();
+    cpu_exec_start(cpu);
 
     do {
         MemTxAttrs attrs;
@@ -2106,6 +2107,7 @@ int kvm_cpu_exec(CPUState *cpu)
         }
     } while (ret == 0);
 
+    cpu_exec_end(cpu);
     qemu_mutex_lock_iothread();
 
     if (ret < 0) {