KVM: x86: Immediately fail the build when possible if required #define is missing

KVM: x86: Immediately fail the build when possible if required #define is missing

Guard usage of the must-be-defined macros in KVM's multi-include headers
with the existing #ifdefs that attempt to alert the developer to a missing
macro, and spit out an explicit #error message if a macro is missing, as
referencing the missing macro completely defeats the purpose of the #ifdef
(the compiler spews a ton of error messages and buries the targeted error
message).

Suggested-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Link: https://patch.msgid.link/20260302212619.710873-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>