kvm tools, qcow: Add support for writing to zero refcount clusters

This patch adds support for writing to zero refcount clusters. Refcount blocks
are cached in like L2 tables and flushed upon VIRTIO_BLK_T_FLUSH and when
evicted from the LRU cache.

With this patch applied, 'qemu-img check' no longer complains about referenced
clusters with zero reference count after

  dd if=/dev/zero of=/mnt/tmp

where '/mnt' is freshly generated QCOW2 image.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>