saving energy while working with qemu

As you know qemu always tends to use 100% of cpu. Qemu console is not available with -nographic option, so saving vm is not possible.

The good news is UNIX (and of course Linux) has a standard mechanism for freezing and unfreezing processes:

$ kill -STOP
(from another shell)

will stop  the process. It will still reside in the  memory (RAM or swap).
To continue process, from the shell where it was started, type

$ fg

There will be no echo, but the command will execute.
If there is an echo, the tty is probably messed up. It can be restored again with

$ stty -brkint -icrnl ixoff -imaxbel iutf8 -isig -icanon -iexten -echo