storage: create: Create files with correct mode
    
    Use correct mode when pre-creating files (for snapshots). The refactor
    changing to storage driver usage caused a regression as some systems
    created the file with 000 permissions forbidding qemu to write the file.
    
    Pass mode to the creating functions to avoid the problem.
    
    Regression since 185e07a5f82bc0692324f3ee13b4816d71b653c1.