KVM: selftests: Add helpers to make Xen-style VMCALL/VMMCALL hypercalls

Add wrappers to do hypercalls using VMCALL/VMMCALL and Xen's register ABI
(as opposed to full Xen-style hypercalls through a hypervisor provided
page).  Using the common helpers dedups a pile of code, and uses the
native hypercall instruction when running on AMD.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230204024151.1373296-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>