Set bitmap size when allocating a bitmap

I began noticing a race when reserving VNC ports as described here

https://www.redhat.com/archives/libvir-list/2010-November/msg00379.html

Turns out that we were not initializing the size field of bitmap
struct when allocating the bitmap.  This subsequently caused
virBitmapSetBit() to fail since bitmap->size is 0, hence we never
actually reserved the port.

src/util/bitmap.c

@@ -73,6 +73,7 @@ virBitmapPtr virBitmapAlloc(size_t size)
         return NULL;
     }
 
+    bitmap->size = size;
     return bitmap;
 }