storage: Refactor iSCSI Source matching

storage: Refactor iSCSI Source matching

Create a separate iSCSI Source matching subroutine. Makes the calling
code a bit cleaner as well as sets up for future patches which need to
do better source hosts[0].name processing/checking.

As part of the effort the logic will be inverted from a multi-level
if statement to a series of single level checks for better readability
and further separation

Signed-off-by: John Ferlan <jferlan@redhat.com>