qemu.tests: Wait for memory balloon back after subtest

Some subtest like reboot will cause memory inside guest ballooned
from original value again. So need wait for the memory size inside
guest adjust to the ballooned value again after the subtest finished.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #136 from ypu/balloon

qemu.tests: Wait for memory balloon back after subtest