Sometimes it's handy to know how many bits are set.

* src/util/bitmap.h (virBitmapCountBits): New prototype.
(virBitmapNextSetBit): Use correct type.
* src/util/bitmap.c (virBitmapNextSetBit): Likewise.
(virBitmapSetAll): Maintain invariant of clear tail bits.
(virBitmapCountBits): New function.
* src/libvirt_private.syms (bitmap.h): Export it.
* tests/virbitmaptest.c (test2): Test it.