linux-user: call cpu_copy under clone_lock

linux-user: call cpu_copy under clone_lock

cpu_copy adds newly created CPU object to container/machine/unattached,
but does it w/o proper locking. As a result when multiple threads create
threads rapidly QEMU may abort with the following message:

  GLib-CRITICAL **: g_hash_table_iter_next: assertion
  'ri->version == ri->hash_table->version' failed

  ERROR:qemu/qom/object.c:1663:object_get_canonical_path_component:
  code should not be reached

E.g. this issue is observed when running glibc test nptl/tst-eintr1.
Move cpu_copy invocation under clone_lock to fix that.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>