cvm guest vsm: allow HV_VP_INDEX_SELF for EnableVpVtl

[sluck-msft]

[smalis-msft]

I did a quick scan of hypercalls that contain a parameter with "vp" in the name. GetVpRegisters, SetVpRegisters, and TranslateVirtualAddress all check for INDEX_SELF already. StartVirtualProcessor doesn't, but it probably doesn't make sense to (how can you start yourself?). EnableVpVtl is the only one that appears to be missing handling.