We were missing check for the fact that the storage driver was found and
in case there is no vbox storage driver available, daemon raised the
following error each start:

  error : virRegisterStorageDriver:592 : driver in
  virRegisterStorageDriver must not be NULL

Fixing this makes the condition unified with networkDriver registration
in vbox as well.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>