libxl: Spice vdagent support for upstream qemu

libxl: Spice vdagent support for upstream qemu

Usage:
- spicevdagent=1|0 (default=0)
Enables spice vdagent. The Spice vdagent is an optional component for
enhancing user experience and performing guest-oriented management
tasks. Its features includes: client mouse mode (no need to grab mouse
by client, no mouse lag), automatic adjustment of screen resolution,
copy and paste (text and image) between client and domU. It also
requires vdagent service installed on domU o.s. to work.

- spice_clipboard_sharing=1|0 (default=0)
Enables Spice clipboard sharing (copy/paste). It requires spicevdagent
enabled.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Acked-by: Ian Campbell <ian.campbell@citrix.com>