[Feature request] Support Virtualbox VISO Virtual ISO to create packer custom CD

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#### Description

Using packer on Windows to build a VM image that requires custom files exposed in an ISO requires packer to actually build an ISO image. Building this ISO image requires external tools not directly available.

VirtualBox has a Virtual ISO feature that can by used to avoid effectively building the ISO: Virtual ISO files, with extension .viso, are mapping instructions used at runtime to expose host files to the VM, is a way similar to the command passed to ISO image building tools.

I have experimented this use with a hand-crafted .viso file and a custom vboxmanage block.

I would rather have packer build the .viso file from the cd_files block, in the same way as with other tools, and inject it into the VM as is already done with actual ISO images build from other tools.

#### Use Case(s)

Building packer custom CD images on Windows with minimal external dependencies, using a Virtualbox feature.
Can be used on any platform, as it does not depend on platform tools, but rather on VirtualBox feature.

#### Potential configuration

same as actual configuration by means of the cd_files block

#### Potential References

https://www.virtualbox.org/manual/ch09.html#viso