block: Use bdrv_do_drain_begin/end in bdrv_drain_all()

block: Use bdrv_do_drain_begin/end in bdrv_drain_all()

bdrv_do_drain_begin/end() implement already everything that
bdrv_drain_all_begin/end() need and currently still do manually: Disable
external events, call parent drain callbacks, call block driver
callbacks.

It also does two more things:

The first is incrementing bs->quiesce_counter. bdrv_drain_all() already
stood out in the test case by behaving different from the other drain
variants. Adding this is not only safe, but in fact a bug fix.

The second is calling bdrv_drain_recurse(). We already do that later in
the same function in a loop, so basically doing an early first iteration
doesn't hurt.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>