release/vm: fix setting NO_ROOT and WITHOUT_QEMU

[brooks]

Summary

To support common shell patterns (test -n and -z) we want NO_ROOT and
WITHOUT_QEMU to be set and not empty of they are set in the Makefile.
Actually do this rather than making them always set (which propogates
to submakes run by mk-vmimage.sh).

Reported by: cperciva
Sponsored by: DARPA, AFRL

[cperciva]

Looks good to me. Slight commit message nit, the issue here is actually .if defined(NO_ROOT) in Makefile logic rather than -n or -z in a shell script. But the same concept applies.