block: Call .drain_begin only once in bdrv_drain_all_begin()

bdrv_drain_all_begin() used to call the .bdrv_co_drain_begin() driver
callback inside its polling loop. This means that how many times it got
called for each node depended on long it had to poll the event loop.

This is obviously not right and results in nodes that stay drained even
after bdrv_drain_all_end(), which calls .bdrv_co_drain_begin() once per
node.

Fix bdrv_drain_all_begin() to call the callback only once, too.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
