Comment the usage of log_call()

Add two comments to the InstallGuestAdditions, explaining why its return
code will not be checked with `log_check_call()` or even manually
grabbing the status informed by `log_call()` itself.

Previously discussed on the #55 issue and 43e3d96 commit.
Merge pull request #57 from myhro/fix-guestadditions

Comment the usage of log_call()