Commands break if a VMX cant be found

[ColdHeat]

mech ls will give an odd error saying "Cannot find a VMX file" if a single VM doesnt have a VMX file.

https://github.com/mechboxes/mech/blob/devel/mech/utils.py#L464

This call is a little premature, we should detect this situation give a meaningful error in the mech ls output.