Add description field to NutanixDriver and update VM creation logic

[tuxtof]

This pull request adds support for customizing the description of Nutanix VMs created by the driver. Users can now specify a description via a new command-line flag, and the driver ensures the description is set appropriately and limited in length.

VM Description Customization:

• Added a new Description field to the NutanixDriver struct to store the VM description.
• Updated the VM creation logic to use the user-provided description from the driver configuration instead of a hardcoded string.
• Introduced a new nutanix-vm-description flag to allow users to specify a custom VM description when creating a VM.
• Enhanced the SetConfigFromFlags method to handle the new description flag, providing a default if not set and truncating descriptions longer than 1000 characters.

fix #168

[wolfganghuse]

LGTM