(maint) centralize system fact stubbing in virtual specs

The specs for the virtual fact on various platforms were duplicating
method stubs for the same fact. This commit removes the stub duplication
and puts the stubs into `before` blocks.
Merge branch 'maint-centralize_virtual_fact_stubs' into 1.6.x
Merge branch '1.6.x' into 1.7.x

Conflicts:
	spec/unit/virtual_spec.rb