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>

src/main.rs

@@ -813,11 +813,13 @@ mod unit_tests {
                 file: None,
                 mode: ConsoleOutputMode::Null,
                 iommu: false,
+                socket: None,
             },
             console: ConsoleConfig {
                 file: None,
                 mode: ConsoleOutputMode::Tty,
                 iommu: false,
+                socket: None,
             },
             devices: None,
             user_devices: None,