KVM: x86: Add function for vectoring error generation

Extract VMX code for unhandleable VM-Exit during vectoring into
vendor-agnostic function so that boiler-plate code can be shared by SVM.

To avoid unnecessarily complexity in the helper, unconditionally report a
GPA to userspace instead of having a conditional entry.  For exits that
don't report a GPA, i.e. everything except EPT Misconfig, simply report
KVM's "invalid GPA".

Signed-off-by: Ivan Orlov <iorlov@amazon.com>
Link: https://lore.kernel.org/r/20241217181458.68690-2-iorlov@amazon.com


[sean: clarify that the INVALID_GPA logic is new]
Signed-off-by: Sean Christopherson <seanjc@google.com>