blockSD: rename mapMetaOffset to getVolumeMetadataSlot

The name of the BlockStorageDomain method mapMetaOffset was not
descriptive enough. The purpose of the method was to return the
metadata slot of a volume, therefore it has been renamed.

Change-Id: I37acb5475eeb8d7b80e8202ec53b5303d63722ae
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26265
Reviewed-by: Dan Kenigsberg <danken@redhat.com>