trace: Update docs to use example events that exist

The events 'qemu_malloc' and 'qemu_free' used in the examples no longer exist,
so use 'qemu_vmalloc' and 'qemu_vfree' instead.
Signed-off-by: NLluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

docs/tracing.txt

@@ -31,8 +31,8 @@ There is a set of static trace events declared in the "trace-events" source
 file.  Each trace event declaration names the event, its arguments, and the
 format string which can be used for pretty-printing:
 
-    qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
-    qemu_free(void *ptr) "ptr %p"
+    qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
+    qemu_vfree(void *ptr) "ptr %p"
 
 The "trace-events" file is processed by the "tracetool" script during build to
 generate code for the trace events.  Trace events are invoked directly from
@@ -40,14 +40,16 @@ source code like this:
 
     #include "trace.h"  /* needed for trace event prototype */
     
-    void *qemu_malloc(size_t size)
+    void *qemu_vmalloc(size_t size)
     {
         void *ptr;
-        if (!size && !allow_zero_malloc()) {
-            abort();
+        size_t align = QEMU_VMALLOC_ALIGN;
+     
+        if (size < align) {
+            align = getpagesize();
         }
-        ptr = oom_check(malloc(size ? size : 1));
-        trace_qemu_malloc(size, ptr);  /* <-- trace event */
+        ptr = qemu_memalign(align, size);
+        trace_qemu_vmalloc(size, ptr);
         return ptr;
     }