Convert guids into mapper devs for extend blockSD

After I1ec54147205992ac130684d01e73cd7aceccad48, which changed
multipath.getMPDevNamesIter() to return full paths,
HSM.extendStorageDomain() guids param should be converted accordingly.

Change-Id: I6da419542d8b4ac61cc9b82c2f84dcf8ce486ced
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1012944
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19659
Reviewed-by: Dan Kenigsberg <danken@redhat.com>