kvm: selftests: conditionally build vm_xsave_req_perm()

vm_xsave_req_perm() is currently defined and used by x86_64 only.
Make it compiled into vm_create_with_vcpus() only when on x86_64
machines. Otherwise, it would cause linkage errors, e.g. on s390x.

Fixes: 415a3c33 ("kvm: selftests: Add support for KVM_CAP_XSAVE2")
Reported-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Tested-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Message-Id: <20220118014817.30910-1-wei.w.wang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>