qemu-thread: fix races on threads that exit very quickly

If we create a thread with QEMU_THREAD_DETACHED mode, QEMU may get a segfault with low probability.

The backtrace is:
   #0  0x00007f46c60291d7 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
   #1  0x00007f46c602a8c8 in __GI_abort () at abort.c:90
   #2  0x00000000008543c9 in PAT_abort ()
   #3  0x000000000085140d in patchIllInsHandler ()
   #4  <signal handler called>
   #5  pthread_detach (th=139933037614848) at pthread_detach.c:50
   #6  0x0000000000829759 in qemu_thread_create (thread=thread@entry=0x7ffdaa8205e0, name=name@entry=0x94d94a "io-task-worker", start_routine=start_routine@entry=0x7eb9a0 <qio_task_thread_worker>,
       arg=arg@entry=0x3f5cf70, mode=mode@entry=1) at util/qemu_thread_posix.c:512
   #7  0x00000000007ebc96 in qio_task_run_in_thread (task=0x31db2c0, worker=worker@entry=0x7e7e40 <qio_channel_socket_connect_worker>, opaque=0xcd23380, destroy=0x7f1180 <qapi_free_SocketAddress>)
       at io/task.c:141
   #8  0x00000000007e7f33 in qio_channel_socket_connect_async (ioc=ioc@entry=0x626c0b0, addr=<optimized out>, callback=callback@entry=0x55e080 <qemu_chr_socket_connected>, opaque=opaque@entry=0x42862c0,
       destroy=destroy@entry=0x0) at io/channel_socket.c:194
   #9  0x000000000055bdd1 in socket_reconnect_timeout (opaque=0x42862c0) at qemu_char.c:4744
   #10 0x00007f46c72483b3 in g_timeout_dispatch () from /usr/lib64/libglib-2.0.so.0
   #11 0x00007f46c724799a in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
   #12 0x000000000076c646 in glib_pollfds_poll () at main_loop.c:228
   #13 0x000000000076c6eb in os_host_main_loop_wait (timeout=348000000) at main_loop.c:273
   #14 0x000000000076c815 in main_loop_wait (nonblocking=nonblocking@entry=0) at main_loop.c:521
   #15 0x000000000056a511 in main_loop () at vl.c:2076
   #16 0x0000000000420705 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4940

The cause of this problem is a glibc bug; for more information, see
https://sourceware.org/bugzilla/show_bug.cgi?id=19951.
The solution for this bug is to use pthread_attr_setdetachstate.

There is a similar issue with pthread_setname_np, which is moved
from creating thread to created thread.

Signed-off-by: linzhecheng <linzhecheng@huawei.com>
Message-Id: <20171128044656.10592-1-linzhecheng@huawei.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
[Simplify the code by removing qemu_thread_set_name, and free the arguments
 before invoking the start routine. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
