util: use "stubDriverType" instead of just "stubDriver"

util: use "stubDriverType" instead of just "stubDriver"

In the past we just kept track of the type of the "stub driver" (the
driver that is bound to a device in order to assign it to a
guest). The next commit will add a stubDriverName to go along with
type, so lets use stubDriverType for the existing enum to make it
easier to keep track of whether we're talking about the name or the
type.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>