qemu.tests: Add a case to query balloon memory size, stop and continue vm from monitor

This case will query balloon memory size then do stop and continue from
monitor. And will check guest status during and after test.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #153 from ypu/balloon_stop_cont

qemu.tests: Add a case to query balloon memory size, stop and continue v...