Fixed generation of log_sink file

Fixed generation of log_sink file

[Bug]i          log_sink file is invalid formated xml
[Cause]         Lxc invoke exit(0) after fork. Exit from child and
                from parent cause redundant log write
[Solution]      Add defintion to CMakeLists.txt
[Verification]  Build, install, build run test with '--log_sink=/tmp/log.xml',
                run 'xmllint --huge --format /tmp/log.xml' (there should
                be no parse errors).
                Note: StartHasStoppedTest can fail.

Change-Id: I9a1d4bee9030eb8e3958ff914ef7cf2dc1a953ce