The function init_blk_migration is better to be called before
set_dirty_tracking as the reasons below.

If we want to track dirty blocks via dirty_maps on a BlockDriverState
when doing live block-migration, its correspoding 'BlkMigDevState' should be
added to block_mig_state.bmds_list first for subsequent processing.
Otherwise set_dirty_tracking will do nothing on an empty list than allocating
dirty_bitmaps for them. And bdrv_get_dirty_count will access the
bmds->dirty_maps directly, then there would be a segfault triggered.

If the set_dirty_tracking fails, qemu_savevm_state_cancel will handle
the cleanup of init_blk_migration automatically.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: chai wen <chaiw.fnst@cn.fujitsu.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>