If a domain is configured to have an egl-headless display and a virtio
video device, virgl will be enabled automatically within the guest, even
if the video device is configured with accel3d='no'.

In this case we should explicitly pass 'virgl=off' to qemu.

See https://bugzilla.redhat.com/show_bug.cgi?id=1791236 for more
information.
Signed-off-by: NJonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>