KVM: nSVM: Raise event on nested VM exit if L1 doesn't intercept IRQs

If L1 doesn't intercept interrupts, then KVM will use vmcb02's V_IRQ
to detect an interrupt window for L1 IRQs.  On a subsequent nested
VM-Exit, KVM might need to copy the current V_IRQ from vmcb02 to vmcb01
to continue waiting for an interrupt window, i.e. if there is still a
pending IRQ for L1.

Raise KVM_REQ_EVENT on nested exit if L1 isn't intercepting IRQs to ensure
that KVM will re-enable interrupt window detection if needed.

Note that this is a theoretical bug because KVM already raises
KVM_REQ_EVENT on each nested VM exit, because the nested VM exit resets
RFLAGS and kvm_set_rflags() raises the KVM_REQ_EVENT unconditionally.

Explicitly raise KVM_REQ_EVENT for the interrupt window case to avoid
having an unnecessary dependency on kvm_set_rflags(), and to document
the scenario.

Signed-off-by: Maxim Levitsky <[email protected]>
[santosh: reworded description as per Sean's v2 comment]
Signed-off-by: Santosh Shukla <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[sean: further massage changelog and comment]
Signed-off-by: Sean Christopherson <[email protected]>

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