Reorganize and fix monitor resume after migration

Reorganize and fix monitor resume after migration

If migration failed in migrate_fd_put_buffer, the monitor may have been
resumed not only in the error path of that function but also once again
in migrate_fd_put_ready which is called unconditionally by
migrate_fd_connect.

Fix this by establishing a cleaner policy: the monitor shall be resumed
when the migration file is closed, either via callback
(migrate_fd_close) or in migrate_fd_cleanup if no file is open (i.e. no
callback invoked).

Reported-By: Michael Tokarev <mjt@tls.msk.ru>
Tested-By: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>