util: bitmap: Add virBitmapToDataBuf that does not allocate the buffer

util: bitmap: Add virBitmapToDataBuf that does not allocate the buffer

Since some functions can be optimized by reusing the buffers that they
already have instead of allocating and copying new ones, lets split
virBitmapToData to two functions where one only converts the data and
the second one is a wrapper that allocates the buffer if necessary.