virt: Expose mount&umount functions for class Nfs

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

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

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

Signed-off-by: Yanbing Du <ydu@redhat.com>
libvirt.test: Re-write virsh_pool.py to cover more commands

    1. Expend case to cover more pool types
    2. Expend case to cover more virsh pool commands
    3. Remove the vol-create/delete part which can be covered in virsh
       volume testing

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