Record number of VCPUs used by a guest and provide an accessor function.

This pull request makes the number of VCPUs used by a particular instance available via the API. This feature supports other existing functions which take the index of a VCPU an input parameter, e.g. vmi_get_vcpureg. The vmi_instance_t structure gained a member, a new accessor function was added, and support for both KVM and Xen was provided.