KVM: selftests: Use vm_create_without_vcpus() in set_boot_cpu_id

Use vm_create_without_vcpus() in set_boot_cpu_id instead of open coding
the equivlant now that the "without_vcpus" variant does
vm_adjust_num_guest_pages().

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