KVM: selftests: Avoid memory allocations when adding vCPU in get-reg-list

Open code adding and doing setup for a vCPU in get-reg-list in order to
avoid the stack allocation that comes with aarch64_vcpu_add_default().
get-reg-list doesn't need to run the vCPU, and so doesn't need the guest
to be backed with memory.  This will allow future cleanup to turn what is
current vm_create() into a barebones helper.

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