Merge branch 'visibleMemory' into 'master'

CA-188697 Fix checkGuestMemory and move guest memory logic to OS classes

This change creates a new property 'visibleMemory' on the OS interface, which returns the amount of memory visible to the guest in MB.
The existing method is now just a wrapper for the new method.
Also remove all concept of unit (and just define it to be in MB) from memory methods.

Additionally the OS class is turned into an abstract base class rather than implementing an interface, and necessary updates are made to ensure unit tests pass etc.

Finally definitions are added for Scientific Linux and CoreOS.

Tested in jobs 1230718 (includes ws12r2core-x64, coreos-stable, rhel, centos and oel guests), and 1230866 (includes sl guest - shows a failure but this appears to be a legitimate issue)

See merge request !134

diff --cc exec/testcases/xenserver/tc/smoketest.py
Simple merge

diff --cc exec/xenrt/lib/xenserver/guest.py
Simple merge

diff --cc exec/xenrt/lib/xenserver/host.py
Simple merge

diff --cc exec/xenrt/objects.py
Simple merge