KVM: use cache=none for shared disk templates

Disable host cache for externally mirrored disks to avoid cache incoherency.
Without this, migrations between the same two nodes may end up in disk
corruption.

This is a runtime override of cluster defaults, mostly a workaround.
Unfortunately, we cannot use cmdlib._CheckHVParams, as we want to check the
hypervisor parameters *in conjunction* with the disk template, which is part of
the instance definition.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>