Replace lsblk fs detection with blkid

[r4victor]

Harden shim's no filesystem detection on dstack-managed volumes:

• Replace lsblk -no FSTYPE that reads from the udev cache with blkid -p superblock that reads device superblock directly.
• Add a readiness gate (waitForBlockDevice + blockDeviceSize): before any format decision the device must be a real block device with non-zero size (syscall via unix.IoctlGetInt, BLKGETSIZE64).
• Introduce 3 extra blkid probes that re-confirm a "no filesystem" verdict with 1s intervals. This is to protect from a hypothetical attach-flakiness.