libvirt.tests: Re-write virsh_pool.py to cover more pool types and 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>
Merge pull request #27 from will-Do/virsh_pool

libvirt.tests: Re-write virsh_pool.py to cover more pool types and commands