Fix registering valid paths in VM tests

Commit 37b56574e20755e4c58370bb18a3d373d4ae7cfc revealed that the code
to get regInfo from /proc/cmdline was broken.  It only happened to
work because the kernel passes the command line to stage 1 through the
environment, so $regInfo was set anyway.
Revert 434260c772622408b1503a4ff2b65bfd900f3c80

There is a bug in unionfs, but it only manifested itself because of
our accidental use of stage-1 libraries in stage-2.