This commit is a "complete" overhaul of libdevuansdk and is a likely
breaking change for all the sdks. They will be updated accordingly.
Notable changes:
* Bootstrap tarballs are now cpio archives.
* Error handling is improved a lot.
* rsync is avoided and cpio is used where necessary.
* debootstrap submodule is removed and system-wide debootstrap is
used instead.
* VMs aren't being built inside an nbd device anymore. They are now
built normally on the filesystem, and copied into a raw image
which is then converted to requested formats (qcow and/or vdi).
* Vagrant build support is completely dropped.
* apt-cache functionality is completely dropped.
* The full-build helper functions are renamed and and separated into
more internal functions/steps.
* Obsolete and unused code is removed.
