KVM: x86: Refactor __kvm_emulate_hypercall() into a macro

Rework __kvm_emulate_hypercall() into a macro so that completion of
hypercalls that don't exit to userspace use direct function calls to the
completion helper, i.e. don't trigger a retpoline when RETPOLINE=y.

Opportunistically take the names of the input registers, as opposed to
taking the input values, to preemptively dedup more of the calling code
(TDX needs to use different registers).  Use the direct GPR accessors to
read values to avoid the pointless marking of the registers as available
(KVM requires GPRs to always be available).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Message-ID: <20241128004344.4072099-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>