fix two parameters of VmParameters in vdsmapi schema

from the comments of VmParameters, the two parameters are #optional

Change-Id: Id9bce789c1d33357e079ff47300a99a59025834e
Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/9112
Reviewed-by: Dan Kenigsberg <danken@redhat.com>