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)).