KVM: PPC: e500mc: Add build-time assert that vcpu is at offset 0

In preparation for moving vcpu allocation to common PPC code, add an
explicit, albeit redundant, build-time assert to ensure the vcpu member
is located at offset 0.  The assert is redundant in the sense that
kvmppc_core_vcpu_create_e500() contains a functionally identical assert.
The motiviation for adding the extra assert is to provide visual
confirmation of the correctness of moving vcpu allocation to common
code.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>