tests: set `volume_cache` 'unsafe'

Set volume_cache to unsafe for CI VMs.
We might be using tmpfs for volume disks soon, therefore 'unsafe' is a
prerequisite for volume_cache

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

Vagrantfile

@@ -134,6 +134,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   # "host doesn't support requested feature: CPUID.01H:EDX.ds [bit 21]"
   config.vm.provider :libvirt do |lv|
     lv.cpu_mode = 'host-passthrough'
+    lv.volume_cache = 'unsafe'
   end
 
   # Faster bootup. Disables mounting the sync folder for libvirt and virtualbox