migration: Clear fd also in error cases

Not clearing the fd and closing the file makes qemu spin using 100%CPU
after incoming migration error.

See for instance bug:
https://bugzilla.redhat.com/show_bug.cgi?id=518032

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
