e2image: Add support for qcow2 format

e2image: Add support for qcow2 format

This commit adds support for exporting filesystem into QCOW2 image
format. Like sparse format this saves space, by writing only necessary
(metadata blocks) into image. Unlike sparse image, QCOW2 image is NOT
sparse, hence does not change its size by copying with not-sparse-aware
tools.

New options '-Q' has been added to tell the e2image to use QCOW2 as an
output image format. QCOW2 supports encryption and compression, however
e2image so far does no support such features, however you can still
scramble filenames with '-s' option.

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