KVM: nVMX: Use vmcs01's controls shadow to check for IRQ/NMI windows at VM-Enter

Use vmcs01's execution controls shadow to check for IRQ/NMI windows after
a successful nested VM-Enter, instead of snapshotting the information prior
to emulating VM-Enter.  It's quite difficult to see that the entire reason
controls are snapshot prior nested VM-Enter is to read them from vmcs01
(vmcs02 is loaded if nested VM-Enter is successful).

That could be solved with a comment, but explicitly using vmcs01's shadow
makes the code self-documenting to a certain extent.

No functional change intended (vmcs01's execution controls must not be
modified during emulation of nested VM-Enter).

Link: https://lore.kernel.org/r/20241101191447.1807602-5-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>