librbd/cache/pwl: handle invalid ImageCacheState json

librbd/cache/pwl: handle invalid ImageCacheState json

get_json_format() and create_image_cache_state() attempt to get
particular keys which could result in an unhandled std::runtime_error
exception.  Conversely, ImageCacheState constructor just swallows that
exception which could leave the newly constructed object incorrectly
initialized.  Avoid doing parsing in the constructor and introduce
init_from_config() and init_from_metadata() methods instead.

While at it, move everything out from under "persistent_cache" key.
Also fix init_state_json_write test case which stopped working now
that types are enforced by json_spirit.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>