xen: Use devclass_find to lookup devclasses in identify.

While here, use driver->name instead of hardcoding the xenpv and
xen_et strings both for devclass_find() and BUS_ADD_CHILD().

Reviewed by: Elliott Mitchell <ehem+freebsd@m5p.com>, imp, royger
Differential Revision: https://reviews.freebsd.org/D35001