migration: Fix use of file after release

qemu_fclose frees the passed file structure, but do_migrate_set_speed
may access it later on. Fix it by setting file NULL in
migrate_fd_cleanup and checking for this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
