rust: make TryFrom macro more resilient

If the enum includes values such as "Ok", "Err", or "Error", the TryInto
macro can cause errors.  Be careful and qualify identifiers with the full
path, or in the case of TryFrom<>::Error do not use the associated type
at all.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api-macros/src/lib.rs

@@ -203,8 +203,8 @@ fn derive_tryinto_body(
     Ok(quote! {
         #(const #discriminants: #repr = #name::#discriminants as #repr;)*;
         match value {
-            #(#discriminants => Ok(#name::#discriminants),)*
-            _ => Err(value),
+            #(#discriminants => core::result::Result::Ok(#name::#discriminants),)*
+            _ => core::result::Result::Err(value),
         }
     })
 }
@@ -236,7 +236,8 @@ fn derive_tryinto_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStrea
         impl core::convert::TryFrom<#repr> for #name {
             type Error = #repr;
 
-            fn try_from(value: #repr) -> Result<Self, Self::Error> {
+            #[allow(ambiguous_associated_items)]
+            fn try_from(value: #repr) -> Result<Self, #repr> {
                 #body
             }
         }