Revert "kvm: selftests: move base kvm_util.h declarations to kvm_util_base.h"

Effectively revert the movement of code from kvm_util.h => kvm_util_base.h,
as the TL;DR of the justification for the move was to avoid #idefs and/or
circular dependencies between what ended up being ucall_common.h and what
was (and now again, is), kvm_util.h.

But avoiding #ifdef and circular includes is trivial: don't do that.  The
cost of removing kvm_util_base.h is a few extra includes of ucall_common.h,
but that cost is practically nothing.  On the other hand, having a "base"
version of a header that is really just the header itself is confusing,
and makes it weird/hard to choose names for headers that actually are
"base" headers, e.g. to hold core KVM selftests typedefs.

For all intents and purposes, this reverts commit
7d9a662e

.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>