Validates the wwn while parsing, error out if it's malformed.

* src/util/util.h: Declare virValidateWWN
* src/util/util.c: Implement virValidateWWN
* src/libvirt_private.syms: Export virValidateWWN.
* src/conf/domain_conf.h: New member 'wwn' for disk def.
* src/conf/domain_conf.c: Parse and format disk <wwn>