driver: add option to make missing drivers a fatal problem

driver: add option to make missing drivers a fatal problem

Currently the driver module loading code does not report an error if the
driver module is physically missing on disk. This is useful for distro
packaging optional pieces. When the daemons are split up into one daemon
per driver, we will expect module loading to always succeed. If a driver
is not desired, the entire daemon should not be installed.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>