schema: allow multiple seclabel for devices in domaincommon.rng

schema: allow multiple seclabel for devices in domaincommon.rng

In our RNG schema we do allow multiple (different) seclabels per-domain,
but don't allow this for devices, yet we neither have a check in our XML parser,
nor in a post-parse callback. In that case we should allow multiple
(different) seclabels for devices as well.