qapi/dump: add allowed-by-guest feature to win-dmp

Commit 1c0e259c5a35 ("dump: make win_dump_available() check vmcoreinfo for a Windows dump header") changed two things in a way that is visible to QMP clients but not to introspection: query-dump-guest-memory-capability now lists win-dmp only for a guest that has published a Windows dump header through the vmcoreinfo device, and dump-guest-memory, which shares win_dump_available(), rejects the format otherwise. Before that, both accepted win-dmp on any x86 machine. A client that wants to select win-dmp automatically therefore cannot trust the capability query on its own: on an older QEMU it reports win-dmp for every x86 guest, Linux ones included, where the resulting dump is useless. libvirt ran into exactly this while picking a format for on_crash and watchdog triggered dumps, and has no way to tell the two behaviours apart. Add an 'allowed-by-guest' feature to the win-dmp member of DumpGuestMemoryFormat so the fixed behaviour becomes discoverable. DumpGuestMemoryFormat is reachable from both query-dump-guest-memory-capability's return type and dump-guest-memory's arguments, so a single flag covers both halves of the change. Where the feature is absent, a reported win-dmp says nothing about the guest, and a client that needs the dump to be loadable afterwards should fall back to elf. CC: Eric Blake <eblake@redhat.com> CC: Markus Armbruster <armbru@redhat.com> CC: "Marc-André Lureau" <marcandre.lureau@redhat.com> Suggested-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Denis V. Lunev <den@openvz.org> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com> Message-ID: <20260731155001.1204103-1-den@openvz.org>