hsm: Rescan multipath before loading lvm cache

lvm cache was loaded before multipath rescan, loading possibly stale
data, since existing devices may disappear and new devices may appear
after multipath rescan.  After lvm cache was loaded, storage domain
cache was refreshed, invoking multipath rescan and invalidating lvm
cache.  Later lvm commands would trigger a second load of the lvm cache.

This patch refreshes storage domain cache before loading lvm cache,
ensuring a fresh cache and saving an unneeded lvm cache reload, which
can be slow when having lot of lvs.

Change-Id: I6ec7beee1db193a47d9e8109929badfd5b322c02
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21567
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>