enhance cloudstack_disk_offering resource with QoS, provisioning, and scope support

[poddm]

Expand cloudstack_disk_offering with QoS, IOPS, and placement controls

Summary

Extends the cloudstack_disk_offering resource to cover the full createDiskOffering/updateDiskOffering API surface — storage/hypervisor QoS, IOPS throttling, domain/zone restriction, and size strictness — plus a rework of the CRUD logic and expanded test coverage.

New schema fields

• IOPS throttling: iops_read_rate, iops_read_rate_max, iops_read_rate_max_length, iops_write_rate, iops_write_rate_max, iops_write_rate_max_length
• Hypervisor byte QoS (nested block): hypervisor { bytes_read_rate, bytes_read_rate_max, bytes_read_rate_max_length, bytes_write_rate, bytes_write_rate_max, bytes_write_rate_max_length }
• Storage IOPS QoS (nested block): storage { min_iops, max_iops, customized_iops, hypervisor_snapshot_reserve }
• Placement / options: domain_id (list), zone_id (list), cache_mode, disk_offering_strictness

Behavior changes

• Removed customized — it's now inferred from disk_size (size set ⇒ fixed offering; size omitted ⇒ customizable). The two are inversely coupled in CloudStack, so exposing both only allowed contradictory config.
• provisioning_type / storage_type changed from Default to Computed (kept ForceNew + validation) so the API-selected value doesn't cause drift.
• cache_mode marked Computed for the same reason.
• zone_id semantics: empty = available in all zones; setting IDs restricts the offering. The internal "all" sentinel used on update to clear a restriction is abstracted away from users.
• Read now handles 404s (removed offerings clear state), and domain_id/zone_id are correctly split from the comma-separated API strings.

Bug fixes (from the original branch)

• Fixed write-rate and hypervisor QoS setters that were all pointing at the read-rate setters.
• Update now actually calls UpdateDiskOffering (previously built params and discarded them) and no longer panics on domain_id/zone_id.

Tests

Acceptance tests: _basic, _customized, _update (expanded to cover cache_mode/display_offering), and new _qos, _options, _domain, _zone.