Prepare for reusing libvirtd source to create other daemons by making
the driver(s) to load conditionally defined by the make rules.

If nothing is set, all drivers will be loaded, ignoring any missing ones
as historically done.

If MODULE_NAME is set only one driver will be loaded and that one must
succeed.
Reviewed-by: NChristophe de Dinechin <dinechin@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>