Force cache=none for KVM when aio=native (#43)
Force cache=none for KVM when aio=native (#43)

Starting with version 2.6, QEMU will fail to start with an error when
aio=native and no cache=none is present. This is documented in
http://wiki.qemu.org/ChangeLog/2.6. QEMU has been complaining about this
since 2.3. Detect this in the hypervisor setup and force the disk_cache
setting, logging it at the same time.

Signed-off-by: Alexandros Kosiaris <fcf6237fcd998caac124d9004b1956dfea72e133@gmail.com>
Reviewed-by: Federico Morg Pareschi <ff862f58af48d157b995cd83870fbfa4aec8edf3@google.com>
mbakke/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,ganeti/ganeti