reports:Fixed bug in br18-chart vms with known OS vs. unknown

Fixed bug in br18 where chart vms with known OS vs. unknown OS.
All OS were defined as Known.
It is now fixed,showing the correct number of known vs.unknown.
Also, Edited the queries of br18 so they will be
more readable and added comments.

Change-Id: Icdecb4e0bc4ac4ce5d6ab2000aa0812aa3ea5c2c
Signed-off-by: Shirly Radco <sradco@redhat.com>
Related-To:  https://bugzilla.redhat.com/1031612