KVM: nVMX: Synthesize nested VM-Exit for supported emulation intercepts

When emulating an instruction on behalf of L2 that L1 wants to intercept,
generate a nested VM-Exit instead of injecting a #UD into L2.  Now that
(most of) the necessary information is available, synthesizing a VM-Exit
isn't terribly difficult.

Punt on decoding the ModR/M for descriptor table exits for now.  There is
no evidence that any hypervisor intercepts descriptor table accesses *and*
uses the EXIT_QUALIFICATION to expedite emulation, i.e. it's not worth
delaying basic support for.

To avoid doing more harm than good, e.g. by putting L2 into an infinite
or effectively corrupting its code stream, inject #UD if the instruction
length is nonsensical.

Link: https://lore.kernel.org/r/20250201015518.689704-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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