s390x/tcg: fix DIAG 308 with > 1 VCPU (MTTCG)

Currently, multi threaded TCG with > 1 VCPU gets stuck during IPL, when
the bios tries to switch to the loaded kernel via DIAG 308.

As run_on_cpu() is used, we run into a deadlock after handling the reset.
We need the iolock (just like KVM).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171116170526.12643-4-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

target/s390x/misc_helper.c

@@ -103,7 +103,9 @@ void HELPER(diag)(CPUS390XState *env, uint32_t r1, uint32_t r3, uint32_t num)
         break;
     case 0x308:
         /* ipl */
+        qemu_mutex_lock_iothread();
         handle_diag_308(env, r1, r3);
+        qemu_mutex_unlock_iothread();
         r = 0;
         break;
     case 0x288: