Refactor iSCSI driver code to facilitate future changes

Refactor iSCSI driver code to facilitate future changes

The following series of patches are adding significant
extra functionality to the iSCSI driver. THe current
internal helper methods are not sufficiently flexible
to cope with these changes. This patch refactors the
code to avoid needing to have a virStoragePoolObjPtr
instance as a parameter, instead passing individual
target, portal and initiatoriqn parameters.

It also removes hardcoding of port 3260 in the portal
address, instead using the XML value if any.

* src/storage/storage_backend_iscsi.c: Refactor internal
  helper methods