Add support for direct setting of cpu_shares when creating a VM

This adds new API field to the Vm structures. The new field
'cpuShares' will override the old 'nice' when used.

Change-Id: I7ef73cf0579a0efd5a31c32638efcc6cdf2447d7
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17528
Reviewed-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>