add as_string() to the Cmdline crate

In some cases, having a String representation of the Linux command line
can be useful. This is the case of vmm-reference which otherwise would require a
conversion dance between string types.

Signed-off-by: Alvise Rigo <a.rigo@virtualopensystems.com>

Author: arigo-vos

src/cmdline/mod.rs

@@ -357,6 +357,33 @@ impl Cmdline {
         }
     }
 
+    /// Returns a String representation of the command line
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use linux_loader::cmdline::*;
+    /// let mut cl = Cmdline::new(20).unwrap();
+    /// cl.insert_str("foo").unwrap();
+    /// cl.insert_init_args("bar").unwrap();
+    /// assert_eq!(cl.as_string().unwrap(), "foo -- bar");
+    /// ```
+    pub fn as_string(&self) -> Result<String> {
+        if self.boot_args.is_empty() && self.init_args.is_empty() {
+            Ok("".to_string())
+        } else if self.boot_args.is_empty() {
+            Err(Error::NoBootArgsInserted)
+        } else if self.init_args.is_empty() {
+            Ok(self.boot_args.to_string())
+        } else {
+            Ok(format!(
+                "{}{}{}",
+                self.boot_args, INIT_ARGS_SEPARATOR, self.init_args
+            )
+            .to_owned())
+        }
+    }
+
     /// Adds a virtio MMIO device to the kernel command line.
     ///
     /// Multiple devices can be specified, with multiple `virtio_mmio.device=` options. This