KVM: x86: Add a helper to dedup reporting of unhandled VM-Exits

KVM: x86: Add a helper to dedup reporting of unhandled VM-Exits

Add and use a helper, kvm_prepare_unexpected_reason_exit(), to dedup the
code that fills the exit reason and CPU when KVM encounters a VM-Exit that
KVM doesn't know how to handle.

Reviewed-by: yaoyuan@linux.alibaba.com
Reviewed-by: Yao Yuan <yaoyuan@linux.alibaba.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030185004.3372256-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>