Move the code to read the binary stats descriptors to the KVM selftests
library. It will be re-used by other tests to check KVM behavior.

No functional change intended.
Reviewed-by: NDavid Matlack <dmatlack@google.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20220613212523.3436117-4-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>