KVM: VMX: Add a helper for NMI handling

Add a helper to handles NMI exit.

TDX handles the NMI exit the same as VMX case.  Add a helper to share the
code with TDX, expose the helper in common.h.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250222014757.897978-15-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>