schema: mark cpuShares optional

'cpuShares' is declared as optional, so it should be prefixed
with an asterisk.

Change-Id: I1d8c7302c966c08090b5d65dfb532a992ece0cd6
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27671
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>