pv: Require matching versions of request and HKD

Enforce that a v{1,2} request also has a v{1,2} hostkey.
This requires to change the signature of Request::add_hostkey to return
a Result.

Co-Developed-by: Timo Keller <tkeller@linux.ibm.com>
Signed-off-by: Timo Keller <tkeller@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Marc Hartmayer <marc@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/pv/src/error.rs

@@ -144,6 +144,9 @@ pub enum Error {
 
     #[error("{}", .0)]
     InvalidHkd(String),
+
+    #[error("All host keys must use the same version (all hybrid or all non-hybrid)")]
+    MixedHostkeyVersions,
 }
 
 // used in macros