conf: Introduce virStoragePoolObjSourceMatchTypeISCSI

conf: Introduce virStoragePoolObjSourceMatchTypeISCSI

In the effort to reduce the virStoragePoolObjSourceFindDuplicate logic,
create a new helper which will handle all the ISCSI type differences.

Alter things just a little bit to return NULL or pool rather than
using breaks and matchpool = pool, then break. Also rather than creating
variables withing the if...else if... conditions, have them all at the
top of the function to make things a bit easier to read.