VCL-1084 - set cache mode for libvirt VMs

libvirt.pm: modified generate_domain_xml: chagned call to is_file_on_local_disk from $self->os->nathost_os->is_file_on_local_disk to $self->vmhost_os->is_file_on_local_disk
diff --git a/managementnode/lib/VCL/Module/Provisioning/libvirt.pm b/managementnode/lib/VCL/Module/Provisioning/libvirt.pm
index 04cccc6..6f2dea8 100644
--- a/managementnode/lib/VCL/Module/Provisioning/libvirt.pm
+++ b/managementnode/lib/VCL/Module/Provisioning/libvirt.pm
@@ -1776,7 +1776,7 @@
 	my $image_type = $self->data->get_vmhost_datastore_imagetype_name();
 	my $vmhost_vmpath = $self->data->get_vmhost_profile_vmpath();
 	my $add_disk_cache = 0;
-	if (! $self->os->nathost_os->is_file_on_local_disk($vmhost_vmpath)) {
+	if (! $self->vmhost_os->is_file_on_local_disk($vmhost_vmpath)) {
 		# set disk cache to none if vmpath on NFS so live migration will work
 		$add_disk_cache = 1;
 	}