Add a new spice chardev to allow arbitrary communication between the
host and the Spice client via the spice server.

Examples:

This allows the Spice client to have a special port for the qemu
monitor:

... -chardev spiceport,name=org.qemu.monitor,id=monitorport
    -mon chardev=monitorport

v2:
- remove support for chardev to chardev linking
- conditionnaly compile with SPICE_SERVER_VERSION
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>