Expose thread_id in info cpus

Based on patch by Glauber Costa:

To allow management applications like libvirt to apply CPU affinities to
the VCPU threads, expose their ID via info cpus. This patch provides the
pre-existing and used interface from qemu-kvm.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
diff --git a/osdep.h b/osdep.h
index 27eedcf..748df54 100644
--- a/osdep.h
+++ b/osdep.h
@@ -130,5 +130,6 @@
 int qemu_madvise(void *addr, size_t len, int advice);
 
 int qemu_create_pidfile(const char *filename);
+int qemu_get_thread_id(void);
 
 #endif