vmm: api: Be more specific on "Still pending remove vcpu" errors

Although the CPU manager gives us a quite descriptive error, on the
application side (the part calling Cloud Hypervisor) we have absolutely
no way to distinguish such error from any other error that may happen
when resizing a VM.

With this in mind, let's be more specific and return a TooManyRequests
(429) error, allowing the caller to have a chance to decide whether they
want to retry the operation or not.

https://datatracker.ietf.org/doc/html/rfc6585#section-4

Signed-off-by: Fabiano Fidêncio <fidencio@northflank.com>

vmm/src/api/http/mod.rs

@@ -52,6 +52,9 @@ pub enum HttpError {
     /// Undefined endpoints
     NotFound,
 
+    /// Too many requests
+    TooManyRequests,
+
     /// Internal Server Error
     InternalServerError,
 
@@ -65,6 +68,7 @@ impl Display for HttpError {
         match self {
             BadRequest => write!(f, "Bad Request"),
             NotFound => write!(f, "Not Found"),
+            TooManyRequests => write!(f, "Too Many Requests"),
             InternalServerError => write!(f, "Internal Server Error"),
             SerdeJsonDeserialize(serde_error) => write!(f, "{}", serde_error),
             ApiError(api_error) => write!(f, "{}", api_error),
@@ -125,6 +129,7 @@ pub trait EndpointHandler {
             Err(e @ HttpError::SerdeJsonDeserialize(_)) => {
                 error_response(e, StatusCode::BadRequest)
             }
+            Err(e @ HttpError::TooManyRequests) => error_response(e, StatusCode::TooManyRequests),
             Err(e) => error_response(e, StatusCode::InternalServerError),
         }
     }