Add missing version comment in onlineCpus

The onlineCpus property of the capabilities command was missing a
comment reporting in which version it was introduced.

Change-Id: I2c544a2c8674461b13aa0ac58abd4d2c768ade0f
Signed-off-by: Vitor de Lima <vdelima@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35100
Reviewed-by: Dan Kenigsberg <danken@redhat.com>