Set bitmap size when allocating a bitmap

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.