having env variables to control aspects of qemu settings should accept the value and the runqemu script should handle the necessary command line syntactic sugar. This would seem to be more intuitive if all you do is read the variable and try to set it. This should be done with all the env variables that runqemu uses that currently expect to have full qemu command lines set.
This is an enhancement, will do it in YP 2.5
Merged into master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=cc283a9028f69dc29f3e28a21be96107be52827b