loopback: do not run in a prefix or vserver

X-Gentoo-Bug: 489370
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=489370
bootmisc: do not run clean_run on VSERVER systems

X-Gentoo-Bug: 489370
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=489370