Move SpiceSession setup to create_spice_session()
    
    Most of the setup (connecting to signals, etc) for the SpiceSession was
    done in create_spice_session(), but some was done afterwards in
    virt_viewer_session_spice_new(). Consolidate all session configuration
    in one place.
    
    In addition to making it easier to maintain, create_spice_session() is
    also called in virt_viewer_session_spice_close(). which results in a
    spice session that is configured slightly differently than the first
    session created in _new(). Consolidating everything in
    create_spice_session() avoids that inconsistency.