qed: Implement .bdrv_drain

qed: Implement .bdrv_drain

The "need_check_timer" is used to clear the "NEED_CHECK" flag in the
image header after a grace period once metadata update has finished. In
compliance to the bdrv_drain semantics we should make sure it remains
deleted once .bdrv_drain is called.

We cannot reuse qed_need_check_timer_cb because here it doesn't satisfy
the assertion.  Do the "plug" and "flush" calls manually.

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Message-id: 1447064214[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>