Allow absolute path to device in `zenfs` utility

[metaspace]

Paths to devices in ZenFS have to be given without leading /dev/ prefix. This is a source of confusion for new users. It is also not documented.

We should allow zenfs utility and RocksDB URI to use absolute paths for devices.

[damien-lemoal]

Yes ! That will also allow using /dev/disk/by-id/xxx as these are stable device names that do not depend on the scan order on boot.

[metaspace]

We might also have issues with using /dev/mapper/... with the current implementation.