libvirt: Re-write virsh_pool test to cover more pool typs and pool commands

1. Wrap mount & umount functions in class Nfs, which can be used by Nfs instance.
2. Add 'iscsi' and 'netfs' type pool define function in class StoragePool.
3. Re-wrirte virsh_pool.py, intent to cover more pool types and pool commands, meanwhile, remove the volume create and delete parts which can be tested in volume testing.