ui: Allow specifying 'rendernode' display option for egl-headless

ui: Allow specifying 'rendernode' display option for egl-headless

As libvirt can't predict which rendernode QEMU would pick, it
won't adjust the permissions on the device, hence QEMU getting
"Permission denied" when opening the DRI device. Therefore, enable
'rendernode' option for egl-headless display type.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1648236

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Message-id: 27f4617f19aa1072114f10f1aa9dd199735ef982.1542362949.git.eskultet@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>