Fix vm creation storage rollback clean

For pools which not allowed create and delete volume,
if create vm fails, its storage volume should not be destoryed.
Put the constant in single file to prevent circular import.
For storage volume deletion, fix deleting vol clear part for readonly pool.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Prevent volume create and delete for certain pool types

As iscsi, scsi, mpath pools do not support create/delete
storage volumes, when creating and deleting volumes,
these pool types should be eleminated.
REF:
    http://libvirt.org/storage.html

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Clear pep8 failure in make check

isoinfo.py, storageservers.py and test_model.py fail in make check
because of pep8, fix it.

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