libvirt-domain: Drop virDomainMigrateCheckNotLocal attribute
    
    Our apibuild.py script does not cope with ATTRIBUTE_NONNULL:
    
    Parse Error: parsing function type, ')' expected
    Got token  ('name', 'char')
    Last token:  ('name', 'char')
    Token queue:  [('op', '*'), ('name', 'dconnuri'), ('sep', ')')]
    Line 3297 end:
    Makefile:2441: recipe for target '../../docs/apibuild.py.stamp' failed
    
    Let's drop it. Moreover, up until e17ae3ccc2dbc1400 where it was
    introduced we did not really care about NULL-ity of dconnuri. And
    moreover the ATTRIBUTE_NONNULL merely checks for static calls
    over NULL, it won't catch the dynamic ones, where a NULL is
    passed by a variable at runtime.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>