qemu-thread: add per-thread atexit functions

qemu-thread: add per-thread atexit functions

Destructors are the main additional feature of pthread TLS compared
to __thread.  If we were using C++ (hint, hint!) we could have used
thread-local objects with a destructor.  Since we are not, instead,
we add a simple Notifier-based API.

Note that the notifier must be per-thread as well.  We can add a
global list as well later, perhaps.

The Win32 implementation has some complications because a) detached
threads used not to have a QemuThreadData; b) the main thread does
not go through win32_start_routine, so we have to use atexit too.

Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: 1417518350[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>