storage: create: Create files with correct mode

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.