fix config adjustment when using data_file with block device

[bensonbbarry]

Summary

Fix two bugs in config adjustment and libvirt XML datastore backend handling.

Changes

Issue 1: dataFilePath undefined when adjust_config is False

• dataFilePath was only assigned inside the if adjust_config block, causing
  an UnboundLocalError when the variable was referenced later with adjust_config=False
• Fixed by initialising dataFilePath = dataFile before the conditional block

Issue 2: Block device backend fails during libvirt XML datastore adjustment

• Code assumed the datastore backend was always a file device
• Added handling for block devices so both file and block backends are
  supported correctly

Testing

• Tested with adjust_config=False — no longer raises UnboundLocalError
• Tested with block device backend — XML adjustment completes successfully

[abbbi]

hi,

thanks, but fails in code format test:

https://github.com/abbbi/virtnbdbackup/actions/runs/28385204851/job/84105978699?pr=317

code should be formatted using black 25.1.0