Revise the preupgrade HIGH risk message.

Modify the high risk check to ask the user to continue. Include the
risks in the output. Use the return value of preupg to determine whether
riskcheck data is available to avoid having to know the path of the
xccdf file.

(cherry picked from commit 19bc6bd3f52e1f5676954b8c04c2775dec02d663)

Related: rhbz#1059447