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)).