drm/vmwgfx: add ioctl for messaging from/to guest userspace to/from host

Up to now, guest userspace does logging directly to host using essentially
the same rather complex port assembly stuff as the kernel.
We'd rather use the same mechanism than duplicate it (it may also change in
the future), hence add a new ioctl for relaying guest/host messaging
(logging is just one application of it).

Signed-off-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>