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: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>