add spice.vgamem option for QXL video memory

When spice.vgamem is set (e.g. 64), uses -device qxl-vga,vgamem_mb=N
instead of -vga qxl (which defaults to 16MB). When null (default),
uses -vga qxl for backwards compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nixos/vms/config.nix

@@ -178,7 +178,7 @@ let
           ${optionalString vmCfg.vnc.enable "-vnc ${vncArgs}"} \
           ${optionalString (vmCfg.spice.enable && vmCfg.spice.passwordFile != null) "-object secret,id=spice-pass-${vmCfg.name},file=${escapeShellArg vmCfg.spice.passwordFile}"} \
           ${optionalString vmCfg.spice.enable "-spice addr=${vmCfg.spice.addr},port=${toString vmCfg.spice.port}${optionalString (vmCfg.spice.passwordFile == null) ",disable-ticketing=on"}${optionalString (vmCfg.spice.passwordFile != null) ",password-secret=spice-pass-${vmCfg.name}"}"} \
-          ${optionalString vmCfg.spice.enable "-vga ${vmCfg.spice.displayDevice}"} \
+          ${optionalString vmCfg.spice.enable (if vmCfg.spice.displayDevice == "qxl" && vmCfg.spice.vgamem != null then "-vga none -device qxl-vga,vgamem_mb=${toString vmCfg.spice.vgamem}" else "-vga ${vmCfg.spice.displayDevice}")} \
           ${optionalString (vmCfg.spice.enable && vmCfg.spice.agent.enable) "-device virtio-serial-pci -chardev spicevmc,id=vdagent,debug=0,name=vdagent -device virtserialport,chardev=vdagent,name=com.redhat.spice.0"} \
           ${# Guest agent channel — prevents qemu-ga from spinning when virtio-win guest tools are installed
             optionalString isWindows "${optionalString (!vmCfg.spice.enable || !vmCfg.spice.agent.enable) "-device virtio-serial-pci"} -chardev socket,path=/tmp/qga-${vmCfg.name}.sock,server=on,wait=off,id=qga0 -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0"} \