Wrong discard_granurality and discard_max_bytes reported in guest OS

[Gorka Eguileor]

Assuming we have configured everything correctly in OpenStack to use discard on Cinder block devices and we have checked that discard works, the wrong values for discard_granularity` and `discard_max_bytes` are reported to the VM's guest Operating System.

We can confirm this by checking the values on the host and the guest and see that they don't match:

- `/sys/block/<dev>/queue/discard_max_bytes`
- `/sys/block/<dev>/queue/discard_granularity`.

The problem is that there is no code in Nova to set these values in libvirt, nor does Cinder or os-brick have any code to detect the right values to set.

The libvirt fuctionality to set `discard_granularity` and `max_unmap_size` already exists: (https://bugzilla.redhat.com/show_bug.cgi?id=1408553.