rust/pv: Add conversion implementations

Add AsRef<[u8]> and TryFrom<Vec<u8>> implementations so that the tag
data can be converted into base64.
See: https://docs.rs/serde_with/latest/serde_with/base64/struct.Base64.html

While at it, fix some typos and simplify the tag extraction.

Suggested-by:  Qi Feng Huo <huoqif@cn.ibm.com>
Acked-by: Qi Feng Huo <huoqif@cn.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/pv/src/error.rs

@@ -79,6 +79,9 @@ pub enum Error {
     #[error("The provided host key document in {0} contains no certificate!")]
     NoHkdInFile(String),
 
+    #[error("Invalid input size ({0}) for boot hdr")]
+    InvBootHdrSize(usize),
+
     // errors from other crates
     #[error(transparent)]
     PvCore(#[from] pv_core::Error),