virttest.test_setup: Shrink the guest mem when host don't have enough huge pages

Shrink the guest mem when host don't have enough huge pages.
Can help the huge page related cases pass in a test loop which may
set up the guest memory very large for other cases in the the
same loop.
If you don't want to shrink the guest memory size please set
hugepage_force_allocate to no.
We also set up another parameter lowest_mem_per_vm for the minimize
memory for one guest. The default value is set to 256M.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>