SEV: why cbitpos is needed instead of just get it by cpuid?

[Wu Zongyong]

Hi,

The @option{cbitpos} is defined with sev-guest object in qemu. And I'm
wondering why this option is needed since we can get the C-bit position
by cpuid.
Besides, the only usecase I found in source code is to check if the
value provided by user is consistent with the return value of cpuid in
function sev_guest_init().
Maybe we can delete this option?

Thanks