virt: Expose mount and umount functions for class Nfs

Signed-off-by: Yanbing Du <ydu@redhat.com>
virttest.libvirt_storage: Add more pool types define-as functions

Signed-off-by: Yanbing Du <ydu@redhat.com>
utils_test.libvirt: Add functions to setup/cleanup nfs/iscsi env and define some pool types

Signed-off-by: Yanbing Du <ydu@redhat.com>
virttest.virsh: Add a series of pool commands

Signed-off-by: Yanbing Du <ydu@redhat.com>