rust/pv_core: Fix the argument order in 'LengthMismatch'

Fix the ordering of the parameters in 'TryFrom<Confidential<Vec<u8> for
Confidential<[u8; N>'. While at it, convert 'LengthMismatch' error to a
named struct.

Fixes: 7608cf2de4 ("rust/confidential: Add `From` and `Into` for confidential byes arrays/vectors")
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pv/src/crypto.rs

@@ -760,7 +760,10 @@ mod tests {
 
         assert!(matches!(
             SymKey::try_from_data(SymKeyType::Aes256Gcm, Confidential::new([0x4u8; 33].into())),
-            Err(Error::PvCore(PvCoreError::LengthMismatch(33, 32)))
+            Err(Error::PvCore(PvCoreError::LengthMismatch {
+                expected: 32,
+                actual: 33
+            }))
         ));
     }
 }