KVM: nVMX: Allow the caller to provide instruction length on nested VM-Exit

KVM: nVMX: Allow the caller to provide instruction length on nested VM-Exit

Rework the nested VM-Exit helper to take the instruction length as a
parameter, and convert nested_vmx_vmexit() into a "default" wrapper that
grabs the length from vmcs02 as appropriate.  This will allow KVM to set
the correct instruction length when synthesizing a nested VM-Exit when
emulating an instruction that L1 wants to intercept.

No functional change intended, as the path to prepare_vmcs12()'s reading
of vmcs02.VM_EXIT_INSTRUCTION_LEN is gated on the same set of conditions
as the VMREAD in the new nested_vmx_vmexit().

Link: https://lore.kernel.org/r/20250201015518.689704-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>