hsm: Check all devices visibility after multipath rescan

After invoking multipath rescan, existing devices may disapper, and
non-existing devices may appear. The orignal code was checking again
only one device after a rescan, possibly returning bogus results.

This patch check now all devices after invoking multipath.rescan().

Change-Id: I46ede1124f4c5f849b040b598b49ee784c540ecc
Relates-To: https://bugzilla.redhat.com/923773
Bug-Url: https://bugzilla.redhat.com/1044068
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21182
Reviewed-by: Ayal Baron <abaron@redhat.com>