In preparation for including the number of dirty pages in the
vfio_get_dirty_bitmap() tracepoint, return the number of dirty pages in
cpu_physical_memory_set_dirty_lebitmap() similar to
cpu_physical_memory_sync_dirty_bitmap().

To avoid counting twice when GLOBAL_DIRTY_RATE is enabled, stash the
number of bits set per bitmap quad in a variable (@nbits) and reuse it
there.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230530180556.24441-2-joao.m.martins@oracle.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>