Replace subprocess backports with the versions from Python 2.7

This fixes some irregularities with call, check_call and check_output.

Resolves: rhbz#1054048

(cherry picked from commit 5e5b9e3a07554128c999e322bf24848dfc382408)
Use the output of losetup to find the loop file

The backing_file sysfs entry doesn't exist on RHEL5-6, so parse some
command output instead.

Related: rhbz#1054048

(cherry picked from commit 19c939012ec837847413b5f5c005b64537363cc3)
Fix a misnamed variable in device_or_mnt

Missed one while switching from argparse to optparse.

Related: rhbz#1054048

(cherry picked from commit 56db6bfaf6e12651a0ba9d60d9e98e8ebe71fb7c)
fix UnboundLocalError with fedup --device

The Great Denaming accidentally made 'fedup.media' and a local variable
named 'media' collide. Rename the local variable to fix that.

(cherry picked from commit 2c723f7aab47b6b6cf21d12a7962f15f50174f07)

Related: rhbz#1054048
New version 0.7.5