Currently, migration will put a BDRV_O_INACTIVE flag
on bs's open_flags until another resume being called. In that case,
any IO from vm or block jobs will cause a qemu crash with an assert
'assert(!(bs->open_flags & BDRV_O_INACTIVE))' failure in bdrv_co_pwritev
function. we hereby disallow block jobs by faking a blocker.
Signed-off-by: wangjian161 <wangjian161@huawei.com>
