Pass volume key as a KeyBuffer

Not for security, but for consistency with the way we handle other
keys, and to move the length check to where it belongs.

Test: create private volume on Cuttlefish
Bug: 147814592
Change-Id: I10fc4896183d050ce25ff174faf78f525cf62930
diff --git a/model/Disk.cpp b/model/Disk.cpp
index b66c336..bfaf2cd 100644
--- a/model/Disk.cpp
+++ b/model/Disk.cpp
@@ -216,7 +216,8 @@
 
     LOG(DEBUG) << "Found key for GUID " << normalizedGuid;
 
-    auto vol = std::shared_ptr<VolumeBase>(new PrivateVolume(device, keyRaw));
+    auto keyBuffer = KeyBuffer(keyRaw.begin(), keyRaw.end());
+    auto vol = std::shared_ptr<VolumeBase>(new PrivateVolume(device, keyBuffer));
     if (mJustPartitioned) {
         LOG(DEBUG) << "Device just partitioned; silently formatting";
         vol->setSilent(true);