libxl: Spice image compression setting support for upstream qemu

Usage:
spice_image_compression=[auto_glz|auto_lz|quic|glz|lz|off]

Specifies what image compression is to be used by spice (if given),
otherwise the qemu default will be used.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
libxl: Spice streaming video setting support for upstream qemu

Usage:
spice_streaming_video=[filter|all|off]

Specifies what streaming video setting is to be used by spice (if given),
otherwise the qemu default will be used.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>