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>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -951,9 +951,11 @@ components:
       properties:
         file:
           type: string
+        socket:
+          type: string
         mode:
           type: string
-          enum: [Off, Pty, Tty, File, Null]
+          enum: [Off, Pty, Tty, File, Socket, Null]
         iommu:
           type: boolean
           default: false