vmm: Only send stdin input to serial/console if it can handle it

Do not send the contents of stdin to the serial or console device if
they're not in tty mode.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/config.rs

@@ -352,6 +352,15 @@ pub enum ConsoleOutputMode {
     File,
 }
 
+impl ConsoleOutputMode {
+    pub fn input_enabled(&self) -> bool {
+        match self {
+            ConsoleOutputMode::Tty => true,
+            _ => false,
+        }
+    }
+}
+
 pub struct ConsoleConfig<'a> {
     pub file: Option<&'a Path>,
     pub mode: ConsoleOutputMode,