domain: Define explicit flags for saved image xml
    
    Commit d2a929d4 (0.9.4) defined virDomainSaveImageGetXMLDesc()'s use
    of @flags as a subset of virDomainXMLFlags, documenting that 2 of the
    3 flags defined at the time would never be valid.  Later, commit
    28f8dfdc (1.0.0) introduced a new flag, VIR_DOMAIN_XML_MIGRATABLE, but
    did not adjust the save image documentation to declare it as invalid.
    Later, commit a67e3872 (3.7.0) blindly copied and pasted the same text
    into virDomainManagedSaveGetXMLDesc.
    
    However, since the flag is not accepted as valid by any of the
    drivers (remote is just passthrough; and qemu is the only supporting
    driver for either API, with support for just VIR_DOMAIN_XML_SECURE),
    it is easier to just define an explicit set of supported flags
    directly related to the save image API rather than trying to borrow
    from live domain API, and risking confusion if even more domain flags
    are added later (in fact, I have an upcoming patch that plans to add
    a new flag to virDomainGetXMLDesc that makes no sense for saved
    images).  We may someday decide that saved images need to support the
    _MIGRATABLE flag, as it is possible to load a saved image with a
    different version of libvirt than the one that created it, but that
    can be a separate patch if it is ever needed.  Meanwhile, it DOES make
    sense to reuse the same flags for SaveImage and for ManagedSave (since
    ManagedSave is really just sugar for creating a normal SaveImage in a
    location controlled by libvirt instead of by the user).
    
    There is no API or ABI impact (since we purposefully used unsigned int
    rather than an enum type in public API, and since the new flag name
    carries the same value as the old reused name).
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>