This should allow the emulated video cards that support hardware
accelerated cursors to relay the cursor drawing to host, possibly
using real hardware cursor. This way the guest and host effectively
share one cursor. Only SDL support is included. Not tested with mice
that report absolute coordinates.

The cursor does not appear in the framebuffer seen by the guest, it's
only drawn on the host. One funny effect is that it's not clipped to
the size of the framebuffer and can stick out of the SDL window.

I think the sdl.c could use a small rewrite to have all cursor hiding
and showing in one place instead of spread across the file.

VNC support would need employing an extension for this, according to
Anthony Liguori. VMware made a documented VNC extension that does it.