migration: use migrate_fd_close in migrate_fd_cleanup

migration: use migrate_fd_close in migrate_fd_cleanup

migrate_fd_cleanup will usually close the file descriptor via
buffered_file_close's call to migrate_fd_close.  However, in the case
of s->file == NULL it is "inlining" migrate_fd_close (almost: there is a
direct close() instead of using s->close(s)).  To fix the inconsistency
and clean up the code, allow multiple calls to migrate_fd_close and use
the function in migrate_fd_cleanup.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>