KVM: x86: Move "emulate hypercall" function declarations to x86.h

Move the declarations for the hypercall emulation APIs to x86.h.  While the
helpers are exported, they are intended to be consumed only by KVM vendor
modules, i.e. don't need to be exposed to the kernel at-large.

No functional change intended.

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>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20241128004344.4072099-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>