Spice: support audio, images and stream compression

Spice: support audio, images and stream compression

This extends the SPICE XML to allow variable compression settings for audio,
images and streaming:
    <graphics type='spice' port='5901' tlsPort='-1' autoport='yes'>
        <image compression='auto_glz'/>
        <jpeg compression='auto'/>
        <zlib compression='auto'/>
        <playback compression='on'/>
    </graphics>

All new elements are optional.