detect-virt: add message at debug level

detect-virt: add message at debug level

Normal users do not have permissions to access /proc/1/root, so
'systemd-detect-virt -r' fails, but the output, even at debug level
is cryptic:

$ SYSTEMD_LOG_LEVEL=debug build/systemd-detect-virt -r
Failed to check for chroot() environment: Permission denied

Let's make this a bit easier to figure out:

$ SYSTEMD_LOG_LEVEL=debug build/systemd-detect-virt -r
Cannot stat /proc/1/root: Permission denied
Failed to check for chroot() environment: Permission denied

I looked over other users of files_same(), and I think in general the message
at debug level is OK for them too.