This simplifies the usage in {libxl,qemu}DomainGetNumaParameters
and it's needed for consistent error reporting in virBitmapFormat.
Also remove the forgotten ATTRIBUTE_NONNULL marker.
We had an easy way to iterate set bits, but not for iterating
cleared bits.
* src/util/virbitmap.h (virBitmapNextClearBit): New prototype.
* src/util/virbitmap.c (virBitmapNextClearBit): Implement it.
* src/libvirt_private.syms (bitmap.h): Export it.
* tests/virbitmaptest.c (test4): Test it.