(maint) Update virtual spec to match dmidecode invocation

A previous commit updated the dmidecode invocation in util/virtual.rb, but it
did not update the corresponding spec test, which stubs the invocation to
return 'something'. This commit updates the virtual spec tests to stub the
updated invocation.
Merge pull request #601 from haus/maint/stable/fix_virtual_specs

(maint) Update virtual spec to match dmidecode invocation