qemu.test: Restructure balloon_check

Now have two different class for Windows and Linux balloon test. Most of the
functions in these classes are same and inherit from class BallooningTest.
They have different error_report and get_memory_status. Also add some different
method to set ballooned memory in cfg files. Now the ballooned memory size also
can be set by a given value or a ratio base on the assign memory value.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.test.cfg: Add a balloon check case with fixed value.

This case follows the following steps:
ballooned memory to 1/2
do guest s3
ballooned memory to 3/4
do guest s4
ballooned memory to 7/8

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