KVM: selftests: Add struct and helpers to wrap binary stats cache

KVM: selftests: Add struct and helpers to wrap binary stats cache

Add a struct and helpers to manage the binary stats cache, which is
currently used only for VM-scoped stats.  This will allow expanding the
selftests infrastructure to provide support for vCPU-scoped binary stats,
which, except for the ioctl to get the stats FD are identical to VM-scoped
stats.

Defer converting __vm_get_stat() to a scope-agnostic helper to a future
patch, as getting the stats FD from KVM needs to be moved elsewhere
before it can be made completely scope-agnostic.

Link: https://lore.kernel.org/r/20250111005049.1247555-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>