vmm: Unix socket backend for serial port

Cloud-Hypervisor takes a path for Unix socket, where it will listen
on. Users can connect to the other end of the socket and access serial
port on the guest.

    "--serial socket=/path/to/socket" is the cmdline option to pass to
cloud-hypervisor.

Users can use socat like below to access guest's serial port once the
guest starts to boot:

    socat -,crnl UNIX-CONNECT:/path/to/socket

Signed-off-by: Praveen K Paladugu <prapal@linux.microsoft.com>

devices/src/legacy/serial.rs

@@ -166,8 +166,8 @@ impl Serial {
         Self::new(id, interrupt, None, state)
     }
 
-    pub fn set_out(&mut self, out: Box<dyn io::Write + Send>) {
-        self.out = Some(out);
+    pub fn set_out(&mut self, out: Option<Box<dyn io::Write + Send>>) {
+        self.out = out;
     }
 
     /// Queues raw bytes for the guest to read and signals the interrupt if the line status would

devices/src/legacy/uart_pl011.rs

@@ -201,8 +201,8 @@ impl Pl011 {
         }
     }
 
-    pub fn set_out(&mut self, out: Box<dyn io::Write + Send>) {
-        self.out = Some(out);
+    pub fn set_out(&mut self, out: Option<Box<dyn io::Write + Send>>) {
+        self.out = out;
     }
 
     fn state(&self) -> Pl011State {