Add support for cpu_cap and cpu_weight Xen params

This patch adds support for Xen's CPU scheduler 'cpu_cap' and
'cpu_weight' parameters.

Ganeti default values (cap: 0=unlimited, weight: 256) are Xen defaults.

cpu_cap is not validated correctly because of actual Ganeti limitation
on checks. I'd need check methods to get beparams or vcpus to get moved
to hvparams.

Signed-off-by: Sébastien Bocahu <sebastien.bocahu@nuxit.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>