imagefile: embed namespace and filesystem inside a file

The resulting file can be used as backing store for a QEMU nvdimm
device. This is based on the approach that is used for the Kata
Container rootfs
(https://github.com/kata-containers/osbuilder/blob/dbbf16082da3de37d89af0783e023269210b2c91/image-builder/image_builder.sh)
and reuses some of the same code, but also differs from that in some
regards:
- The start of the partition is aligned a multiple of the 2MiB
  huge page size (kata-containers/runtime#2262 (comment)).
- The size of the QEMU object is the same as the nominal size of the
  file. In Kata Containers the size is a fixed 128MiB
  (kata-containers/osbuilder#391 (comment)).

go.mod

@@ -27,7 +27,7 @@ require (
 	github.com/prometheus/common v0.4.1 // indirect
 	github.com/prometheus/procfs v0.0.5 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
-	github.com/stretchr/testify v1.4.0 // indirect
+	github.com/stretchr/testify v1.4.0
 	go.uber.org/atomic v1.4.0 // indirect
 	go.uber.org/multierr v1.2.0 // indirect
 	go.uber.org/zap v1.11.0 // indirect