block/qcow2: Add qcow2_refcount_area()

block/qcow2: Add qcow2_refcount_area()

This function creates a collection of self-describing refcount
structures (including a new refcount table) at the end of a qcow2 image
file. Optionally, these structures can also describe a number of
additional clusters beyond themselves; this will be important for
preallocated truncation, which will place the data clusters and L2
tables there.

For now, we can use this function to replace the part of
alloc_refcount_block() that grows the refcount table (from which it is
actually derived).

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 20170613202107[email protected]
Signed-off-by: Max Reitz <[email protected]>