Simplify argument parsing of vmctl stop

Simplify argument parsing of vmctl stop

The previous argument parsing logic had at least three bugs:
a copy-paste error led to an off-by-one and a printf "%s" NULL,
as reported by Preben Guldberg.  A previous commit led to a dead
else branch and a use of uninitialized.  This can all be avoided
by reworking the logic so as to be readable.

Prompted by a diff from Preben

ok dv