KVM: selftests: Expand set of APIs for pinning tasks to a single CPU

KVM: selftests: Expand set of APIs for pinning tasks to a single CPU

Expand kvm_pin_this_task_to_pcpu() into a set of APIs to allow pinning a
task (or self) to a CPU (any or specific).  This will allow deduplicating
code throughout a variety of selftests.

Opportunistically use "self" instead of "this_task" as it is both more
concise and less ambiguous.

Link: https://lore.kernel.org/r/20250626001225.744268-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>