selinux: Resolve resource leak using the default disk label

selinux: Resolve resource leak using the default disk label

Commit id a994ef2d1 changed the mechanism to store/update the default
security label from using disk->seclabels[0] to allocating one on the
fly. That change allocated the label, but never saved it.  This patch
will save the label. The new virDomainDiskDefAddSecurityLabelDef() is
a copy of the virDomainDefAddSecurityLabelDef().
(cherry picked from commit 05cc03518987fa0f8399930d14c1d635591ca49b)

Conflicts:
	src/conf/domain_conf.h