This adds two properties to the QcowInspector that makes it able to
indicate whether the file specifies a backing_file or data_file in the
header. Both conditions are considered unsafe for our usage. To
ease checking of this condition, a classmethod is added that takes
a local filename and digests just enough of the file to assert that
both conditions are false.
Change-Id: Iaf86b525397d41bd116999cabe0954a0a7efac65
Related-Bug: #2059809
(cherry picked from commit ae536bb394793c9a7a219cb498e03d5c81dbbbb7)
(cherry picked from commit 2eba54e0821106097dfeceb424e53943fd090483)
(cherry picked from commit 89dbbc838d606f461087e1494d19ddbcf9db0a38)
(cherry picked from commit 4860024286)
f32d5b8ad8