Since .io_flush() is no longer called we do not need
qemu_gluster_aio_flush_cb() anymore.  It turns out that qemu_aio_count
is unused now and can be dropped.

Thanks to Bharata B Rao <bharata@linux.vnet.ibm.com> for catching a
build failure with CONFIG_GLUSTERFS_DISCARD, which has been fixed.

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