release/vm: fix setting NO_ROOT and WITHOUT_QEMU

To support common shell patterns (test -n and -z) we want NO_ROOT and
WITHOUT_QEMU to be set and not empty in mk-vmimage.sh if they are set at
all in the Makefile.

Actually do this rather than making them always set (which propogates to
submakes run by mk-vmimage.sh and unconditionally enables NO_ROOT with
undesirable side effects such as installing a /METALOG in vm images).

Reported by: cperciva
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D49544