CP-9273: correctly check for overflow in fill

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: fix size for create

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: high-level unit tests for create

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: high-level unit tests for create

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>