Add "port" parameter to the "graphics" schema

[khmarochos]

Summary

Add support for custom port configuration in VNC graphics for libvirt domains. This feature allows users to specify a fixed port number for VNC connections instead of relying on automatic port assignment.

Changes

• Add port parameter to graphics schema: Optional string parameter with numeric validation
• Implement validation constraints: Port parameter can only be used with VNC graphics type and requires autoport = false
• Update domain XML generation: Properly handles port configuration in libvirt XML, defaults to -1 when not specified
• Add comprehensive tests: Covers both successful configuration and validation error cases

Usage Example

resource "libvirt_domain" "example" {
  name = "example-domain"
  graphics {
    type     = "vnc"
    autoport = false
    port     = "5900"
  }
}

Validation Rules

• Port parameter is only valid when type = "vnc"
• When port is specified, autoport must be false
• Port value must be a valid numeric string
• Configuration errors provide clear validation messages

Test Coverage

• TestAccLibvirtDomain_GraphicsVNCPort: Tests successful VNC port configuration
• TestAccLibvirtDomain_GraphicsVNCPortValidation: Tests validation scenarios including invalid type, autoport conflicts, and non-numeric values

Backward Compatibility

This change is fully backward compatible. Existing configurations continue to work unchanged, and the new port parameter is optional.

[khmarochos]

The related feature request is #1182.