feat(image_to_vm): Refactor to make adding/changing vm formats easier.

The old script was heading towards spaghetti code realm. This breaks up
all the image variations such as hybrid MBR, OEM packages, etc into
configuration options and small functions that actually do the work.

All this is in the new vm_image_util.sh library but the command line
parsing and overall procedure remains in image_to_vm.sh

As part of this we gain support for putting some qemu options in a
config file as well as Xen virtual machines using pygrub and pvgrub.

Lots of generally unused options have been removed to simplify things
and keep output file names consistent.