spice/vnc: client migration.

spice/vnc: client migration.

Handle spice client migration, i.e. inform a spice client connected
about the new host and connection parameters, so it can move over the
connection automatically.

The monitor command has a not-yet used protocol argument simliar to
set_password and expire_password commands.  This allows to add a simliar
feature to vnc in the future.  Daniel Berrange plans to work on this.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>