If no -t option is specified, bool writethrough stayed uninitialised.
Initialise it as false, which makes cache=writeback the default cache
mode.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>