util: promote qemu_egl_rendernode_open() to libqemuutil

vhost-user-gpu will share the same code to open a DRM node.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180713130916.4153-20-marcandre.lureau@redhat.com>

[ kraxel: buildfix: util/drm.o must be CONFIG_OPENGL not CONFIG_LINUX ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>