vm-build: Add script to build in a microvm

This script uses the `ocivm` tool to launch a QEMU micro VM to build
the operating system.  This is necessary to produce an SELinux-enabled
system, since container runtimes interfere with the SELinux policy
build and filesystem labeling processes.

vm-build.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+exec ./ocivm.sh make O=/tmp/build 'IMAGESDIR=${PWD}/images'