1. When windows guest mem is small(e.g.1024M), calculated mem
min_size will be larger than ori_mem if ratio=0.5, which will
lead balloon cases fail.
2. When do balloon operation in linux guest, we should consider
crashkernel value as well,leaving enough memory for guest to
avoid guest OOM.

So modify the formula and leave a balloon buffer(300M in default)
for guest min_sz.
Signed-off-by: NLi Jin <lijin@redhat.com>