runqemu: check permissions of available render nodes as well as their presence

runqemu: check permissions of available render nodes as well as their presence

qemu itself is not helpful when render nodes exist, but can't be opened:

qemu-system-x86_64: egl: render node init failed

To fix this, users likely need to

 * modprobe vgem (presence when physical graphic card is absent or has a driver without
support for render nodes, such as many older cards found in server machines)

 * add their user to "render" group to write to /dev/dri/renderD* (permissions)

With this change runqemu should print hints for the above as appropriate from probing the nodes.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>