If QEMU is compiled with graphical window support, and is started without `-nodefaults`, then you can press ctrl-alt-2 to switch to the monitor, type "sendkey ctrl-alt-delete", then press ctrl-alt-1 to switch back to the emulated display.
Common QEMU frontends may have a UI button or keyboard sequence to do this for you (they connect to the monitor backend and send the same command, either as text or as JSON (in "qmp" mode)).
Does anyone know what how to type special keys, like ctrl-alt-delete, in qemu/quickemu?