hax: unbreak accelerator cpu code after cpus.c split

during my split of cpus.c, code line
"current_cpu = cpu"
was removed by mistake, causing hax to break.

This commit fixes the situation restoring it.

Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Fixes: e92558e4bf8059ce4f0b310afe218802b72766bc
Signed-off-by: Claudio Fontana <cfontana@suse.de>
Message-Id: <20201016080032.13914-1-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/hax-cpus.c

@@ -38,6 +38,7 @@ static void *hax_cpu_thread_fn(void *arg)
     qemu_thread_get_self(cpu->thread);
 
     cpu->thread_id = qemu_get_thread_id();
+    current_cpu = cpu;
     hax_init_vcpu(cpu);
     cpu_thread_signal_created(cpu);
     qemu_guest_random_seed_thread_part2(cpu->random_seed);