qemu-ga: document vsock-listen in the man page

Although qemu-ga has supported vsock since 2016 it was not documented on
the man page.

Also add the socket address representation to the qga --help output.

Fixes: 586ef5de
       ("qga: add vsock-listen method")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

docs/interop/qemu-ga.rst

@@ -36,13 +36,14 @@ Options
 .. option:: -m, --method=METHOD
 
   Transport method: one of ``unix-listen``, ``virtio-serial``, or
-  ``isa-serial`` (``virtio-serial`` is the default).
+  ``isa-serial``, or ``vsock-listen`` (``virtio-serial`` is the default).
 
 .. option:: -p, --path=PATH
 
   Device/socket path (the default for virtio-serial is
   ``/dev/virtio-ports/org.qemu.guest_agent.0``,
-  the default for isa-serial is ``/dev/ttyS0``)
+  the default for isa-serial is ``/dev/ttyS0``). Socket addresses for
+  vsock-listen are written as ``<cid>:<port>``.
 
 .. option:: -l, --logfile=PATH
 

qga/main.c

@@ -234,7 +234,9 @@ QEMU_COPYRIGHT "\n"
 "  -p, --path        device/socket path (the default for virtio-serial is:\n"
 "                    %s,\n"
 "                    the default for isa-serial is:\n"
-"                    %s)\n"
+"                    %s).\n"
+"                    Socket addresses for vsock-listen are written as\n"
+"                    <cid>:<port>.\n"
 "  -l, --logfile     set logfile path, logs to stderr by default\n"
 "  -f, --pidfile     specify pidfile (default is %s)\n"
 #ifdef CONFIG_FSFREEZE