enable write zeroes optimisations

enable write zeroes optimisations

This enable write zeroes optimisation inside guest.

(I have tested them with qcow2,raw (ext4|xfs) , zfs|iscsi and nfs too.

No impact if the block driver don't support it yet (like rbd)

https://lists.nongnu.org/archive/html/qemu-devel/2014-04/msg00009.html

> a) mkfs.ext4 -E lazy_itable_init=0,lazy_journal_init=0 /dev/vdX
>
> QCOW2         [off]     [on]     [unmap]
> -----
> runtime:       14secs    1.1secs  1.1secs
> filesize:      937M      18M      18M
>
> iSCSI         [off]     [on]     [unmap]
> ----
> runtime:       9.3s      0.9s     0.9s
>
> b) dd if=/dev/zero of=/dev/vdX bs=1M oflag=direct
>
> QCOW2         [off]     [on]     [unmap]
> -----
> runtime:       246secs   18secs   18secs
> filesize:      51G       192K     192K
> throughput:    203M/s    2.3G/s   2.3G/s
>
> iSCSI*        [off]     [on]     [unmap]
> ----
> runtime:       8mins     45secs   33secs
> throughput:    106M/s    1.2G/s   1.6G/s
> allocated:     100%      100%     0%

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>