scripts/runqemu: move render nodes check to runqemu from selftest

This will produce a more useful hint for those setting
up or testing virgl headless:

runqemu - ERROR - No render nodes found in /dev/dri: ['by-path', 'card0']. If /dev/dri/renderD* is absent due to lack of suitable GPU, 'modprobe vgem' will create one suitable for mesa llvmpipe software renderer.

as qemu itself isn't helpful:

alex@Zen2:/srv/storage/alex/yocto/build-64-alt$ qemu-system-x86_64 -display egl-headless
qemu-system-x86_64: egl: no drm render node available
qemu-system-x86_64: egl: render node init failed

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>