* Fix: Add disk space checking for OPNsense VM FreeBSD image decompression
- Add check_disk_space() function to verify available storage
- Check for 20GB before download and 15GB before decompression
- Provide clear error messages showing available vs required space
- Add proper error handling for unxz decompression failures
- Clean up compressed .xz file after decompression to save space
- Add progress messages for download and decompression steps
Fixes issue where script fails at line 611 with 'No space left on device'
when /tmp directory lacks sufficient space for ~10-15GB decompressed image.
* Increase OPNsense VM disk size from 10GB to 20GB
- Provides more space for system updates, logs, and package installations
- 20GB is a more appropriate size for OPNsense production use
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