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

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: 586ef5dee77180fc32e33bc08051600030630239
       ("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>