Wrap mount&umount functions

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

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