KVM: x86: Add emulation status for unhandleable exception vectoring

Add emulation status for unhandleable vectoring, i.e. when KVM can't
emulate an instruction because emulation was triggered on an exit that
occurred while the CPU was vectoring an event.  Such a situation can
occur if guest sets the IDT descriptor base to point to MMIO region,
and triggers an exception after that.

Exit to userspace with event delivery error when KVM can't emulate
an instruction when vectoring an event.

Signed-off-by: Ivan Orlov <iorlov@amazon.com>
Link: https://lore.kernel.org/r/20241217181458.68690-3-iorlov@amazon.com


[sean: massage changelog and X86EMUL_UNHANDLEABLE_VECTORING comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>