tdx::linux::device::is_available should check for user privilege

The `tdx::linux::device::is_available_function()` function checks whether the Linux KVM device exists in a TD guest. This operation needs `sudo` privileges to succeed, but currently throws an error. Instead, this function should check for permissions if a user is running the command from an unprivileged account.

_Originally posted by @sandlbn in https://github.com/IntelLabs/tdx-workload-attestation/pull/1#discussion_r2065359377_

Start by reading tdx::linux::device::is_available_function() and trace how it checks for the Linux KVM device. Compare its behavior for privileged and unprivileged users, then verify that an unprivileged account receives a permission check rather than an error when the device is available.

Written by the indexing model from the issue text.