qemu/migration: fix the double free problem on from_src_file

In load_snapshot, mis->from_src_file is freed twice, the first free is by
qemu_fclose, the second is by migration_incoming_state_destroy and
it causes Illegal instruction exception. The fix is just to remove the
first free.

This problem is found by qemu-iotests case 068 since commit
"660819b migration: shut src return path unconditionally". The error is:
068 1s ... - output mismatch (see 068.out.bad)
    --- tests/qemu-iotests/068.out	2017-05-06 01:00:26.417270437 +0200
    +++ 068.out.bad	2017-06-03 13:59:55.360274640 +0200
    @@ -6,6 +6,8 @@
     QEMU X.Y.Z monitor - type 'help' for more information
     (qemu) savevm 0
     (qemu) quit
    +./common.config: line 107: 242472 Illegal instruction     (core dumped) ( if [ -n "${QEMU_NEED_PID}" ]; then
    +    echo $BASHPID > "${QEMU_TEST_DIR}/qemu-${_QEMU_HANDLE}.pid";
    +fi; exec "$QEMU_PROG" $QEMU_OPTIONS "$@" )
     QEMU X.Y.Z monitor - type 'help' for more information
    -(qemu) quit
    -*** done
    +(qemu) *** done

Signed-off-by: QingFeng Hao <haoqf@linux.vnet.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
