char: Fix socket with "type": "vsock" address

Watch this:

    $ qemu-system-x86_64 -nodefaults -S -display none -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 91, "minor": 8, "major": 2}, "package": " (v2.8.0-1195-gf84141e-dirty)"}, "capabilities": []}}
    { "execute": "qmp_capabilities" }
    {"return": {}}
    { "execute": "chardev-add", "arguments": { "id": "chr0", "backend": { "type": "socket", "data": { "addr": { "type": "vsock", "data": { "cid": "CID", "port": "P" }}}}}}
    Aborted (core dumped)

Crashes because SocketAddress_to_str() is blissfully unaware of
SOCKET_ADDRESS_KIND_VSOCK.  Fix that.  Pick the output format to match
socket_parse(), just like the existing formats.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1490895797-29094-3-git-send-email-armbru@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
