glib versions prior to 2.31.0 require an explicit g_thread_init() call
to enable multi-threading.

Failure to initialize threading causes glib to take single-threaded code
paths without synchronization.  For example, the g_slice allocator will
crash due to race conditions.

Fix this for all QEMU tool programs (qemu-nbd, qemu-io, qemu-img) by
moving the g_thread_init() call from vl.c:main() into a new
osdep.c:thread_init() constructor function.

thread_init() has __attribute__((constructor)) and is automatically
invoked by the runtime during startup.

We can now drop the "simple" trace backend's g_thread_init() call since
thread_init() already called it.

Note that we must keep coroutine-gthread.c's g_thread_init() call which
is located in a constructor function.  There is no guarantee for
constructor function ordering so thread_init() may only be called later.
Reported-by: NMario de Chenno <mario.dechenno@unina2.it>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>