run: Introduce --use-malloc-perturb specific qemu flag

Setting the environment variable MALLOC_PERTURB_ to
a non zero value [1] helps to find out some subtle
memory allocation problems, and it is being used in
our test grid. However, by default, the virt test
runner doesn't use it.

Add the new, qemu test specific --use-malloc-perturb flag
and set it to 'yes' (use the env variable) by default.
This way we're being more strict, which makes sense
in the context of a test suite. If people want to double
check the behavior of the test without the flag, just
use '--use-malloc-perturb no'.

[1] http://udrepper.livejournal.com/11429.html

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>