[installer] Use "dm reboot" instead of dmctl.

We are removing the dmctl virtual device. This should use the standard
command flow through the "dm" shell command to reboot the device.

Issue: ZX-3403
Test:
fx build build/images:installer
fx mkzedboot -i /dev/sda
** Boot device and on shell do: **
sh install/install-fuchsia

Change-Id: Icea6a5d6cf2c92a658ff6637b6726db0a88d5541