Fix segfault on migration completion

A simple migration reproduces it:

1. Start the source VM with:

   # qemu [...] -S

2. Start the destination VM with:

   # qemu <source VM cmd-line> -incoming tcp:0:4444

3. In the source VM:

   (qemu) migrate -d tcp:0:4444

4. The source VM will segfault as soon as migration completes (might not
   happen in the first try)

What is happening here is that qemu_file_put_notify() can end up closing
's->file' (in which case it's also set to NULL). The call stack is rather
complex, but Eduardo helped tracking it to:

select loop -> migrate_fd_put_notify() -> qemu_file_put_notify() ->
buffered_put_buffer() -> migrate_fd_put_ready() ->
migrate_fd_completed() -> migrate_fd_cleanup().

To be honest, it's not completely clear to me in which cases 's->file'
is not closed (on error maybe)? But I doubt this fix will make anything
worse.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
