set proper migration status on ->write error (v5)

If ->write fails, declare migration status as MIG_STATE_ERROR.

Also, in buffered_file.c, ->close the object in case of an
error.

Fixes "migrate -d "exec:dd of=file", where dd fails to open file.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>