virsh_snapshot_edit: Resolve test errors

Test was failing because the 're.search()' wasn't stripping off the
newline character from the after_xml, thus not allowing the match check
to determine if the <description> changed. Changed the error processing
to list the 15 or so relevant lines from the snapshot dump xml that
could be different if the re.search() fails.

Remove the extraneous .strip() from the after_xml creation since it's
repeated in the snap_xml_compare() method when doing the comparison
with the pre_xml. Also changed the logic on error path to just print
some different lines rather than printing both whole pre/after xml
and forcing an eyeball comparison for why the test failed.