vm.py: Fix comment regarding MOM and KSM

Fixed the comment's typo and grammar mistake.

Change-Id: I2e59d230536e176891f25d3bb19d11b90958190e
Signed-off-by: Allon Mureinik <amureini@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18680
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>