kvm tools: Add VIRTIO_BLK_T_FLUSH feature to handle flush operation from VM

The virtual machine calls 'sync' when the machine
is halted. Adding the virtio flush feature will
ensure that the data is synced on to disk before
the virtual machine is halted. This is needed to
ensure the intigrity of the data.

Signed-off-by: Prasad Joshi <prasadjoshi124@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>