KIWI plugin to provide self contained build support

 The KIWI boxed plugin provides support for self contained building of images
 based on fast booting VM images.
 .
 Specifically, this plugin adds the `system boxbuild` subcommand to KIWI as an
 alternative to the `system build` subcommand. This command downloads a
 prebuilt VM image and runs the given KIWI command line inside the VM. The
 operating system powering the VM image can be selected using the `--box`
 argument. The plugin then runs `kiwi system build` inside the VM with all
 arguments that are passed to the `kiwi system boxbuild` command after the
 `--` sequence.
 .
 Example: kiwi system boxbuild --box ubuntu -- <standard KIWI build arguments>
 .
 The boxbuild process requires either `qemu-system-x86` or `qemu-system-arm`,
 depending on which target architecture is being built.