Fix tests on aosp_cf_x86_64_only_phone

Our nested virtualization check failed on this target. Move to a
prefix check to support this device and any other ones that appear.

Also extract the code to do the check to a library, rather than
changing in two places.

Bug: 236922543
Test: Presubmit
Change-Id: Id1d424f5f107e2098d89ca2ea8089ba956f212ca
diff --git a/libs/nested_virt/Android.bp b/libs/nested_virt/Android.bp
new file mode 100644
index 0000000..e364a2d
--- /dev/null
+++ b/libs/nested_virt/Android.bp
@@ -0,0 +1,18 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+rust_library {
+    name: "libnested_virt",
+    crate_name: "nested_virt",
+    srcs: ["src/lib.rs"],
+    edition: "2018",
+    rustlibs: [
+        "libanyhow",
+        "librustutils",
+    ],
+    apex_available: [
+        "com.android.compos",
+        "com.android.virt",
+    ],
+}
diff --git a/libs/nested_virt/src/lib.rs b/libs/nested_virt/src/lib.rs
new file mode 100644
index 0000000..ab1f06a
--- /dev/null
+++ b/libs/nested_virt/src/lib.rs
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//! Detection for nested virtualization.
+
+use anyhow::Result;
+use rustutils::system_properties;
+
+/// Return whether we will be running our VM in a VM, which causes the nested VM to run very slowly.
+pub fn is_nested_virtualization() -> Result<bool> {
+    // Currently nested virtualization only occurs when we run KVM inside the cuttlefish VM.
+    // So we just need to check for vsoc.
+    if let Some(value) = system_properties::read("ro.build.product")? {
+        // Fuzzy matching to allow for vsoc_x86, vsoc_x86_64, vsoc_x86_64_only, ...
+        Ok(value.starts_with("vsoc_x86"))
+    } else {
+        Ok(false)
+    }
+}