Remove VM generation dead-code.

Remove VM generation dead-code.

This code duplicates the (gnu system image) and (gnu build image) code. Using
VM for image generation is not needed, not portable and really slow. Remove
all the VM image generation code to make sure that only the image API is used.

* gnu/build/vm.scm: Remove it. Move the qemu-command procedure to ...
* gnu/build/marionette.scm: ... here.
* gnu/local.mk (GNU_SYSTEM_MODULES): Adapt it.
* tests/modules.scm: Ditto.
* gnu/tests/install.scm: Ditto.
* gnu/system/vm.scm: Adapt it and remove expression->derivation-in-linux-vm,
qemu-img, system-qemu-image/shared-store and system-docker-image procedures.
* doc/guix.texi (G-Expressions): Adapt it.