Summary: Pass StoragePoolType that maps to libvirt in cleanupDisk

Description: When selecting a storage adaptor, cleanupDisk assumes that
libvirt is being used. Therefore, we pass a StoragePoolType that maps to
libvirt. This is the only place in LibvirtComputingResource where the
StoragePoolType can't be pulled from somewhere else.

BUG-ID: CLOUDSTACK-1011
Signed-off-by: Marcus Sorensen <marcus@betterservers.com>