build: Move image building to cmd/build.go

Move the image building code from QEMU driver to cmd/build.go. This is
the first step into killing the QEMU dependency for building images and
use whatever hypervisor is available on the system.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #113 from penberg/build-code-movement

build: Move image building to cmd/build.go