KVM: use cache=unsafe

KVM: use cache=unsafe

When running with raw image files, we're not passing in any cache= parameter,
making KVM use the default. Unfortunately, the default is horribly slow and
way too secure about data which doesn't matter for our build use case.

So instead, we should just use cache=unsafe, rendering flush requests from the
guest as nops, making builds faster by many times.

Signed-off-by: Alexander Graf <agraf@suse.de>