Rework the agl-demo-preload feature to specify a new demo-specific
PREFERRED_RPROVIDER for the new virtual/low-can-dev-mapping virtual
package. This enables using the sllin0 interface on the CES demo
setup as the low-speed interface.
Bug-AGL: SPEC-3076
Change-Id: I872137d8acb1f5ec5df9eac92f25953b28128b36 Signed-off-by: Scott Murray <scott.murray@konsulko.com>