qcow2: Cache refcount blocks during snapshot creation

The really time consuming part of snapshotting is to adjust the reference count
of all clusters. Currently after each adjusted cluster the refcount block is
written to disk.

Don't write each single byte immediately to disk but cache all writes to the
refcount block and write them out once we're done with the block.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>