virttest.staging.utils: Add four functions related with host memory status check

Add these four function in virttest for memory related test cases

read_from_vmstat: Use for reading host memory status such as nr_mlock
read_from_smaps: Use for reading process status like AnonHugePages
get_buddy_info: Get the fragement status in host. Will helpful for
                the huge page related function to decide if there
                is enough space for the test.
read_from_numa_maps: Use for reading process numa status like the pages
                     use in each node and informations like this.

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