Move setup of libvirt disk cachemode into imagebackend module
Move setup of libvirt disk cachemode into imagebackend module

There is alot of repeated code setting the disk cache mode.
Move this into the imagebackend module so that it can be
done in one place

Change-Id: I7bff6b0b2e7e40a5aef3126634ca814082f8cf4f
Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

n0ano/ganttclient