virttest: add new function to utils_misc

get guest disk free space and memory free size

Signed-off-by: Xu Tian <xutian@redhat.com>
@will-Do 
sorry, I  misunderstand staging module before. Please forget above comment.
double check freememtotal function found  that used for get free memory in host, here I guest free memory in guest. so that not suite to use it here.

Thanks your comment.

Xu