block: perform zero-detection during copy-on-read Copy-on-Read populates the image file with data read from a backing image. In order to avoid bloating the image file when all zeroes are read we should scan the buffer and perform an optimized zero write operation. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>