hsm: fix isoprefix KeyError for inactive domains

In a recent change f9cf58b (Make getRepoStats() a hsm method) a
new regression was introduced that causes a KeyError on isoprefix
for inactive domains.

In this patch:
* fix the isoprefix KeyError for inactive domains
* fix indentation for a related block of code

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1020543
Change-Id: Ice78a4d652d495aff8672524fcd80b7306013928
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20250
Reviewed-by: Dan Kenigsberg <danken@redhat.com>