If your virtual system is running under qmeu then you can access the qemu monitor to interact with that os, various things can be achieved such as sending keystrokes, sending nmi, changing state of the vm, snapshotting, add/remove devices etc.
To gain access to the monitor
ctrl-alt-2
There is a good description of the various options in QEMU-Buch / QEMU-Book, the book is in German but there is an English translation.
Notes on various technologies and other cool stuff I am playing around with. My own technical stream of consciousness.
