libextr2fs: handle short reads/writes while creating the qcow file

libextr2fs: handle short reads/writes while creating the qcow file

This issue was flagged by Coverity, although its analysis was
incorrect.  This isn't actually a memory overrun / security issue, but
rather a functional correctness issue since POSIX allows reads and
writes to be partially completed, and in those cases qcow2_copy_data()
could result in a corrutped qcow2 file.

Addresses-Coverity-Bug: 1531830
Signed-off-by: Theodore Ts'o <tytso@mit.edu>