Fix default value of security label 'relabel' attribute

Fix default value of security label 'relabel' attribute

When no <seclabel> is present in the XML, the virDomainSeclabelDef
struct is left as all zeros. Unfortunately, this means it gets setup
as type=dynamic, with relabel=no, which is an illegal combination.

Change the 'bool relabel' attribute in virDomainSeclabelDef to
the inverse 'bool norelabel' so that the default initialization
is sensible

* src/conf/domain_conf.c, src/conf/domain_conf.h,
  src/security/security_apparmor.c, src/security/security_selinux.c:
  Replace 'relabel' with 'norelabel'