qcow2: Options' documentation fixes

In order to choose the cache sizes we need to know how they relate to

the amount of allocated space.

The amount of virtual disk that can be mapped by the L2 and refcount

The part of the virtual disk that can be mapped by the L2 and refcount

caches (in bytes) is:

   disk_size = l2_cache_size * cluster_size / 8

   disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits

With the default values for cluster_size (64KB) and refcount_bits

(16), that is

(16), this becomes:

   disk_size = l2_cache_size * 8192

   disk_size = refcount_cache_size * 32768

   l2_cache_size = disk_size_GB * 131072

   refcount_cache_size = disk_size_GB * 32768

QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount

cache of 256KB (262144 bytes), so using the formulas we've just seen

we have

For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual

image size (given that the default cluster size is used):

   1048576 / 131072 = 8 GB of virtual disk covered by that cache

    262144 /  32768 = 8 GB

   8 GB / 8192 = 1 MB

The refcount cache is 4 times the cluster size by default. With the default

cluster size of 64 KB, it is 256 KB (262144 bytes). This is sufficient for

8 GB of image size:

   262144 * 32768 = 8 GB

How to configure the cache sizes

   memory as possible to the L2 cache before increasing the refcount

   cache size.

 - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"

   can be set simultaneously.

Unlike L2 tables, refcount blocks are not used during normal I/O but

only during allocations and internal snapshots. In most cases they are

accessed sequentially (even during random guest I/O) so increasing the

@item cache-size

The maximum total size of the L2 table and refcount block caches in bytes

(default: 1048576 bytes or 8 clusters, whichever is larger)

(default: the sum of l2-cache-size and refcount-cache-size)

@item l2-cache-size

The maximum size of the L2 table cache in bytes

(default: 4/5 of the total cache size)

(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever

is larger; otherwise, as large as possible or needed within the cache-size,

while permitting the requested or the minimal refcount cache size)

@item refcount-cache-size

The maximum size of the refcount block cache in bytes

(default: 1/5 of the total cache size)

(default: 4 times the cluster size; or if cache-size is specified, the part of

it which is not used for the L2 cache)

@item cache-clean-interval

Clean unused entries in the L2 and refcount caches. The interval is in seconds.