qemu.tests: Fix the wrong balloon type in cfg

After s4 step in balloon_fix_value, the guest memory is back to
original value. So the type for next balloon should be evict but not
enlarge.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #76 from ypu/balloon_fix

qemu.tests: Fix the wrong balloon type in cfg