Add support for VM builder specific options from veewee

Provide handling for VM provider specific (virtualbox and vmfusion) options within veewee templates. This addresses problems with converting the ESXi veewee templates (mitchellh/veewee-to-packer#6), and ensures other templates that use virtualbox settings are processed appropriately. Behavior of these conversions was verified with current veewee trunk.

Also added a handler to ignore any KVM specific settings, in case those are added to a given template. Note that while a `:kvm` item is defined for a definition file, the veewee code does not appear to us it.

Verified proper behavior of this code against the ESXi 5.1 Veewee template, and the scientificlinux-6.1-i386 templates.