conf: gfx: egl-headless: Introduce a new <gl> subelement

conf: gfx: egl-headless: Introduce a new <gl> subelement

Unlike with SPICE and SDL which use the <gl> subelement to enable OpenGL
acceleration, specifying egl-headless graphics in the XML has
essentially the same meaning, thus in case of egl-headless we don't have
a need for the 'enable' element attribute and we'll only be interested
in the 'rendernode' one further down the road.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>