block: fix bdrv_ioctl called from coroutine

When called from a coroutine, bdrv_ioctl must be asynchronous just like
e.g. bdrv_flush.  The code was incorrectly making it synchronous, fix
it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>