virgl/vtest: add vtest driver

virgl/vtest: add vtest driver

virgl/vtest is a swrast driver that allows the
virgl acceleration to be tested without having
a virtual machine.

The backend has a unix socket server that
this connects to.

This is run by setting
LIBGL_ALWAYS_SOFTWARE=y
GALLIUM_DRIVER=virpipe

In this mode all renderering is sent over
a socket to the remote renderer, and the
results are readback and copies to the screen
using drisw. This works well enough to develop
new features and to help debug.

Signed-off-by: Dave Airlie <airlied@redhat.com>