Import http routes from Cloud Hypervisor project

Import vmm/src/api/http.rs from Cloud Hyerpvisor project, commit
345c922cb9a88183e2da9d29230ecb945b0a6452 with following changes:
1) use generic type for handler argument.
2) remove server thread relative code.
3) add unit test cases.
4) refine for better code reuse.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

src/common/mod.rs

@@ -67,6 +67,21 @@ impl Display for ConnectionError {
     }
 }
 
+/// Errors pertaining to `HttpRoute`.
+#[derive(Debug)]
+pub enum RouteError {
+    /// Handler for http routing path already exists.
+    HandlerExist(String),
+}
+
+impl Display for RouteError {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
+        match self {
+            RouteError::HandlerExist(p) => write!(f, "handler for {} already exists", p),
+        }
+    }
+}
+
 /// Errors pertaining to `HttpServer`.
 #[derive(Debug)]
 pub enum ServerError {