virtio-mem: Fix the bitmap index of the section offset

virtio-mem: Fix the bitmap index of the section offset

vmem->bitmap indexes the memory region of the virtio-mem backend at a
granularity of block_size. To calculate the index of target section offset,
the block_size should be divided instead of the bitmap_size.

Fixes: 2044969f0b ("virtio-mem: Implement RamDiscardManager interface")
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20221216062231.11181-1-chenyi.qiang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: David Hildenbrand <david@redhat.com>