KVM: selftests: Move the guts of kvm_hypercall() to a separate macro

Extract the guts of kvm_hypercall() to a macro so that Xen hypercalls,
which have a different register ABI, can reuse the VMCALL vs. VMMCALL
logic.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230204024151.1373296-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>