qemu-img bench: Fix uninitialised writethrough mode

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: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
