Use str type for vcpuQuota to prevent xml-rpc errors

Change-Id: Id7c38759ee65e58530c03751ca12706396697166
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28960
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>