Add SPICE support to add_client monitor command

Add SPICE support to add_client monitor command

With the acceptance of some new APIs to libspice-server.so it
is possible to add support for SPICE to the 'add_client'
monitor command, bringing parity with VNC. Since SPICE can
use TLS or plain connections, the command also gains a new
'tls' parameter to specify whether TLS should be attempted
on the injected client sockets.

This new feature is only enabled if building against a
libspice-server >= 0.10.1

* qmp-commands.hx: Add 'tls' parameter & missing doc for
  'skipauth' parameter
* monitor.c: Wire up SPICE for 'add_client' command
* ui/qemu-spice.h, ui/spice-core.c: Add qemu_spice_display_add_client
  API to wire up from monitor

[1] http://cgit.freedesktop.org/spice/spice/commit/server/spice.h?id=d55b68b6b44f2499278fa860fb47ff22f5011faa
    http://cgit.freedesktop.org/spice/spice/commit/server/spice.h?id=bd07dde530d9504e1cfe7ed5837fc00c26f36716

Changes in v3:
 - Added 'optional' flag to new parameters documented
 - Added no-op impl of qemu_spice_display_add_client when
   SPICE is disabled during build

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>