KVM: selftests: Add VM creation helper that "returns" vCPUs

Add a VM creator that "returns" the created vCPUs by filling the provided
array.  This will allow converting multi-vCPU tests away from hardcoded
vCPU IDs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>