tests/test-bdrv-drain: bdrv_drain_all() works in coroutines now

tests/test-bdrv-drain: bdrv_drain_all() works in coroutines now

Since we use bdrv_do_drained_begin/end() for bdrv_drain_all_begin/end(),
coroutine context is automatically left with a BH, preventing the
deadlocks that made bdrv_drain_all*() unsafe in coroutine context. Now
that we even removed the old polling code as dead code, it's obvious
that it's compatible now.

Enable the coroutine test cases for bdrv_drain_all().

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