scylla_util.py: resolve /dev/root to get actual device on aws
scylla_util.py: resolve /dev/root to get actual device on aws

When psutil.disk_paritions() reports / is /dev/root, aws_instance mistakenly
reports root partition is part of ephemeral disks, and RAID construction will
fail.
This prevents the error and reports correct free disks.

Fixes #8055

Closes #8040

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla