e2image: Support for conversion QCOW2 image into raw

e2image: Support for conversion QCOW2 image into raw

This commit adds support for converting QCOW2 image created previously
with e2image into raw image. The QCOW2 image is detected automatically,
so there is not new option. Just use following command:

  e2image -r image.qcow image.raw

No that this tool is aimed to quickly convert qcow2 image created with
e2image into raw image. In order to improve speed we are doing some
assumption I believe might not be true for regular qcow2 images. So it
was not tested with regular QCOW2 images and it might not work with
them. The intention of this tool is only convert images previously
created by e2image.

Note that there is nothing special with QCOW2 images created by e2images
and it can be used with tools like qemu-img, or qemu-nbd without any
problems.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>