snapshot: Make virDomainMomentObjListGetNames more generic

snapshot: Make virDomainMomentObjListGetNames more generic

Rather than hard-coding the snapshot filter bit values into the
generic code, add another layer of indirection: callers must map which
of their public filter bits correspond to supported moment bits, then
pass two separate flags (the ones translated for moment code to
operate on, and the remaining ones for the filter callback to operate
on).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>