Filter directory in storage volume listing

When listing storage volumes, directory will be listed

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
fix test case for volume filtering

Test mock model passed an unsupported type when volume create,
fix it.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>