KVM: x86: Try to unprotect and retry on unhandleable emulation failure

If emulation is "rejected" by check_emulate_instruction(), try to
unprotect and retry instruction execution before reporting the error to
userspace.  Currently, check_emulate_instruction() never signals failure
when "unprotect and retry" is possible, but that will change in the
future as both VMX and SVM will reject emulation due to coincident
exception vectoring.  E.g. if there is a write to a shadowed page table
when vectoring an event, then unprotecting the gfn and retrying the
instruction will allow the guest to make forward progress in most cases,
i.e. will allow the vCPU to keep running instead of returning an error to
userspace.

This ensures that the subsequent patches won't make KVM exit to
userspace when handling an intercepted #PF during vectoring without
checking whether unprotect and retry is possible.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ivan Orlov <iorlov@amazon.com>
Link: https://lore.kernel.org/r/20241217181458.68690-4-iorlov@amazon.com
[sean: massage changelog to clarify this is a nop for the current code]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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