hw/loongarch: Remove default enable with VIRTIO_VGA device

hw/loongarch: Remove default enable with VIRTIO_VGA device

For virtio VGA deivce libvirt will select VIRTIO_VGA firstly rather than
VIRTIO_GPU, VIRTIO_VGA device supports frame buffer however it requires
legacy VGA compatible support. Frame buffer area 0xa0000 -- 0xc0000
conflicts with low memory area 0 -- 0x10000000.

Here remove default support for VIRTIO_VGA device, VIRTIO_GPU is prefered
on LoongArch system. For frame buffer video card support, standard VGA can
be used.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20240823073050.2619484-1-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>