VM: Handle not found file descriptor (#14479)

This simply adds a check for whether a file descriptor with the provided
name exists and return an error if it doesn't.
The motivation for this was the fact that we found out we were using
mismatched names for block devices file descriptors on create/removal,
which caused removal to not take effect (refer to
#14461 (comment)).
This will be fixed by #14461 and
then this PR can be merged to detect future instances of this same
problem.

lxd/instance/drivers/qmp/commands.go

@@ -225,6 +225,8 @@ func (m *Monitor) RemoveFDFromFDSet(name string) error {
 		return fmt.Errorf("Failed to query fd sets: %w", err)
 	}
 
+	found := false
+
 	for _, fdSet := range resp.Return {
 		for _, fd := range fdSet.FDs {
 			fields := strings.SplitN(fd.Opaque, ":", 2)
@@ -237,6 +239,8 @@ func (m *Monitor) RemoveFDFromFDSet(name string) error {
 			}
 
 			if opaque == name {
+				found = true
+
 				args := map[string]any{
 					"fdset-id": fdSet.ID,
 				}
@@ -249,6 +253,11 @@ func (m *Monitor) RemoveFDFromFDSet(name string) error {
 		}
 	}
 
+	// Return an error if no fd matched the provided name.
+	if !found {
+		return api.StatusErrorf(http.StatusNotFound, "No fd with name %q", name)
+	}
+
 	return nil
 }