ui: deprecate "password" option for SPICE server

With the new "password-secret" option, there is no reason to use the old
inecure "password" option with -spice, so it can be deprecated.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210311114343.439820-4-berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>