KVM: selftests: Read binary stat data in lib

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

Also opportunistically remove an unnecessary calculation with
"size_data" in stats_test.

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220613212523.3436117-6-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>