migration/multifd: Ensure we're not given a socket for file migration

When doing migration using the fd: URI, QEMU will fetch the file
descriptor passed in via the monitor at
fd_start_outgoing|incoming_migration(), which means the checks at
migration_channels_and_transport_compatible() happen too soon and we
don't know at that point whether the FD refers to a plain file or a
socket.

For this reason, we've been allowing a migration channel of type
SOCKET_ADDRESS_TYPE_FD to pass the initial verifications in scenarios
where the socket migration is not supported, such as with fd + multifd.

The commit decdc76772 ("migration/multifd: Add mapped-ram support to
fd: URI") was supposed to add a second check prior to starting
migration to make sure a socket fd is not passed instead of a file fd,
but failed to do so.

Add the missing verification and update the comment explaining this
situation which is currently incorrect.

Fixes: decdc76772 ("migration/multifd: Add mapped-ram support to fd: URI")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240315032040.7974-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/migration.c

@@ -166,9 +166,9 @@ static bool transport_supports_seeking(MigrationAddress *addr)
     }
 
     /*
-     * At this point, the user might not yet have passed the file
-     * descriptor to QEMU, so we cannot know for sure whether it
-     * refers to a plain file or a socket. Let it through anyway.
+     * At this point QEMU has not yet fetched the fd passed in by the
+     * user, so we cannot know for sure whether it refers to a plain
+     * file or a socket. Let it through anyway and check at fd.c.
      */
     if (addr->transport == MIGRATION_ADDRESS_TYPE_SOCKET) {
         return addr->u.socket.type == SOCKET_ADDRESS_TYPE_FD;