seclabel: Do not output relabel attribute for type 'none'

seclabel: Do not output relabel attribute for type 'none'

Security label type 'none' requires relabel to be set to 'no' so there's
no reason to output this extra attribute.  Moreover, since relabel is
internally stored in a negative from (norelabel), the default value for
relabel would be 'yes' in case there is no <seclabel> element in domain
configuration.  In case VIR_DOMAIN_SECLABEL_DEFAULT turns into
VIR_DOMAIN_SECLABEL_NONE, we would incorrectly output relabel='yes' for
seclabel type 'none'.