CA-148436: Fix xencert metadata tests for ISCSI SR

Changes part of this commit:
* SR ATTACH (Switch to a different method to 'Create_SR')
* SR SCAN (Blacklist 'metadata_of_pool' & 'snapshot_time')
* SR PROBE (Log successful parameter validation)

Additionally, white-space errors were corrected in the affected methods.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>