CONFIG_LINUX_AIO is an implementation detail of raw-posix.c.  Don't
mention CONFIG_LINUX_AIO in blockdev.c.  Let block drivers decide what
to do with BDRV_O_NATIVE_AIO.  They may print an error if it is
unsupported.

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