Can't boot Qubes VMs error "Document is empty, line 1, column 1"

If You will recive such error in QUBES during vm start (www is name of VM):

You need to run:
find /var/lib/qubes -name 'firewall.xml'

and delete all files that You mentioned command will find. Of course it is good to verify contents of those files before deletion ;). In my case files were empty and that was the root cause of that issue.