Doc: add work around to handle NFS root squash problem

Tested:
    1. make
    2. nfs pool and vm creation
Default NFS server export path is configured as root squash,
mapping root user to nobody.
This results:
1. Root user cannot step into mount point if export path
   does not allow other to read/execute.
   So create volume will fail.
2. Even with other permission open,
   owner/group of volume created by root is still nobody/nogrp,
   and qemu permission is denied on such img.
This work around instruct user to enable squash to given libvirt user
to address the above problems.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>