Skip over offline storage devices when mounting isos

When mounting ISOs via Instance > Settings > Media, was getting the following error that is telling me about an iSCSI pool that is not related to the VM I was changing:

``Requested operation is not valid: storage pool `vmpool-iscsi' is not active``

Tracked it down to the following area this pull request fixes, which tells ``mount_iso`` to skip over offlined storaged pools.

To reproduce, create a pool against a host, then stop it.  Then try and attach an ISO to a VM.  If you look at ``get_iso_media`` which retrieves the list of ISOs to available to mount, it does the same thing.