qcow2: add qcow2_cache_discard

qcow2: add qcow2_cache_discard

Whenever l2/refcount table clusters are discarded from the file we can
automatically drop unnecessary content of the cache tables. This reduces
the chance of eviction useful cache data and eliminates inconsistent data
in the cache with the data in the file.

Signed-off-by: Pavel Butsykin <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 20170918124230[email protected]
Signed-off-by: Max Reitz <[email protected]>