This fixes the bug introduced by this commit ad54ae80.
The bdrv_aio_ioctl() still could return null and we should return an error
in that case.

Cc: qemu-stable@nongnu.org
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>