qemu: enable virglrenderer and glx options subject to 'opengl' DISTRO_FEATURE

Note that to actually use accelerated GL passthrough, there are two options

1) a suitable frontend need to be also enabled - gtk+ and SDL both seem to work well.
Previously I struggled to make SDL work, but now it seems fine.

2) it is also possible to render off-screen with -display egl-headless option,
and see the output with a VNC viewer (for which, qemu needs to be started
with a VNC server):

$ runqemu kvm egl-headless publicvnc

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>