hsm: validate storage pool in prepareImage

If the storage pool is specified (non-blank) we should validate that
we're connected.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1130995
Change-Id: Ia85c2b2acedf7bdbadd930596987cc4828e24ddd
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32502
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>