e2image, libext2fs: check for corrupted qcow2 image

If the qcow2 image is corrupted, qcow2_write_image() will now return
an indication of this to e2image (the only current user of
qcow2_write_image).

Also fix how e2image prints an error message it can't understand the
qcow2 image.

Addresses-Coverity-Bug: 1297511
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
3 files changed