xenapi: set vcpu cap to ensure weight is applied

There is a bug in XenServer 6.1 and 6.2 that means the vcpu weight
we set is ignored if the cap is not also set.

Setting the cap to its default of 0, meaning no cap, to ensure the
weight value is applied.

Fixes bug 1213960
Change-Id: I367a9afe188734cf5e95c01fc014b9044003d6e3
Merge "xenapi: set vcpu cap to ensure weight is applied"