qemu.tests: Small bug fix for ksm_base

Correct the type of shared_mem in script. And enlarge the guest_script_overhead
in cfg file to ignore the memory leak lead in guest which will lead to the memory
allocate speed slow and make case failed in the memory allocate step.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #109 from ypu/ksm

qemu.tests: Small bug fix for ksm_base