rust/pv: Fix padding of retrievable private keys

Fix the padding of keys that are smaller than the architected slot for
the retrievable key. Previously the keys where appended with zeros.
However, processing software expects a left-padding.
Affected key types:
  * Ed448
  * SecP521

While at it, fix documentation in the retrievable key struct.

Fixes: fd024387d7 ("rust/pv: Retrievable secrets support")
Acked-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pv/src/error.rs

@@ -117,6 +117,9 @@ pub enum Error {
         exp: String,
     },
 
+    #[error("Invalid data from OpenSSL")]
+    InvalSslData,
+
     // errors from other crates
     #[error(transparent)]
     PvCore(#[from] pv_core::Error),