test-bdrv-drain: Test BlockDriver callbacks for drain

This adds a test case that the BlockDriver callbacks for drain are
called in bdrv_drained_all_begin/end(), and that both of them are called
exactly once.

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