Give virDomainDef parser & formatter their own flags
    
    The virDomainDefParse* and virDomainDefFormat* methods both
    accept the VIR_DOMAIN_XML_* flags defined in the public API,
    along with a set of other VIR_DOMAIN_XML_INTERNAL_* flags
    defined in domain_conf.c.
    
    This is seriously confusing & error prone for a number of
    reasons:
    
     - VIR_DOMAIN_XML_SECURE, VIR_DOMAIN_XML_MIGRATABLE and
       VIR_DOMAIN_XML_UPDATE_CPU are only relevant for the
       formatting operation
     - Some of the VIR_DOMAIN_XML_INTERNAL_* flags only apply
       to parse or to format, but not both.
    
    This patch cleanly separates out the flags. There are two
    distint VIR_DOMAIN_DEF_PARSE_* and VIR_DOMAIN_DEF_FORMAT_*
    flags that are used by the corresponding methods. The
    VIR_DOMAIN_XML_* flags received via public API calls must
    be converted to the VIR_DOMAIN_DEF_FORMAT_* flags where
    needed.
    
    The various calls to virDomainDefParse which hardcoded the
    use of the VIR_DOMAIN_XML_INACTIVE flag change to use the
    VIR_DOMAIN_DEF_PARSE_INACTIVE flag.