docs: Add caution in docs that not all --display's works with virtio

[ymo-4]

I haven't found this in docs. In my opinion it should be on the first pages since using something other then virtio-vga disables virgl so no gpu acceleration =(. For linux guest, virtio-vga works with everything except spice|spice-app, and for windows guest virtio-vga works only with cocoa|sdl|spice-app (but not spice). For *bsd and macos guests virtio wont work at all

quickemu/quickemu

Lines 898 to 940 in 790484c

	case ${guest_os} in
	*bsd) DISPLAY_DEVICE="VGA";;
	linux_old|solaris) DISPLAY_DEVICE="vmware-svga";;
	linux)
	case ${display} in
	none|spice|spice-app) DISPLAY_DEVICE="virtio-gpu";;
	*) DISPLAY_DEVICE="virtio-vga";;
	esac;;
	macos)
	# qxl-vga and VGA supports seamless mouse and sane resolutions if only
	# one scanout is used. '-vga none' is added to the QEMU command line
	# to avoid having two scanouts.
	DISPLAY_DEVICE="VGA";;
	windows|windows-server)
	# virtio-gpu "works" with gtk but is limited to 1024x1024 and exhibits other issues
	# https://kevinlocke.name/bits/2021/12/10/windows-11-guest-virtio-libvirt/#video
	case ${display} in
	gtk|none|spice) DISPLAY_DEVICE="qxl-vga";;
	cocoa|sdl|spice-app) DISPLAY_DEVICE="virtio-vga";;
	esac;;
	*) DISPLAY_DEVICE="qxl-vga";;
	esac
	# Map Quickemu $display to QEMU -display
	case ${display} in
	gtk) DISPLAY_RENDER="${display},grab-on-hover=on,zoom-to-fit=off,gl=${gl}";;
	none|spice) DISPLAY_RENDER="none";;
	sdl) DISPLAY_RENDER="${display},gl=${gl}";;
	spice-app) DISPLAY_RENDER="${display},gl=${gl}";;
	*) DISPLAY_RENDER="${display}";;
	esac
	# https://www.kraxel.org/blog/2021/05/virtio-gpu-qemu-graphics-update/
	if [ "${gl}" == "on" ] && [ "${DISPLAY_DEVICE}" == "virtio-vga" ]; then
	if [ "${QEMU_VER_SHORT}" -ge 61 ]; then
	DISPLAY_DEVICE="${DISPLAY_DEVICE}-gl"
	else
	DISPLAY_DEVICE="${DISPLAY_DEVICE},virgl=on"
	fi
	echo -n " - Display: ${display^^}, ${DISPLAY_DEVICE}, GL (${gl}), VirGL (on)"
	else
	echo -n " - Display: ${display^^}, ${DISPLAY_DEVICE}, GL (${gl}), VirGL (off)"
	fi