ui: introduce "password-secret" option for VNC servers

Currently when using VNC the "password" flag turns on password based
authentication. The actual password has to be provided separately via
the monitor.

This introduces a "password-secret" option which lets the password be
provided up front.

  $QEMU --object secret,id=vncsec0,file=passwd.txt \
        --vnc localhost:0,password-secret=vncsec0

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210311114343.439820-2-berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>