KVM: selftests: Add test case for MMIO during vectoring on x86

Extend the 'set_memory_region_test' with an x86-only test case which
covers emulated MMIO during event vectoring error handling. The test case

1) Sets an IDT descriptor base to point to an MMIO address
2) Generates a #GP in the guest
3) Verifies userspace gets the correct exit reason, suberror code, and
   GPA in internal.data[3]

Opportunistically add a definition for a non-canonical address to
processor.h so that the source of the #GP is somewhat self-documenting,
and so that future tests don't have to reinvent the wheel.

Signed-off-by: Ivan Orlov <iorlov@amazon.com>
Link: https://lore.kernel.org/r/20241217181458.68690-8-iorlov@amazon.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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