hostdev: add virObject field to virHostdevManager struct

hostdev: add virObject field to virHostdevManager struct

Commit 6b306d66 converted virHostdevManager to a virObject, but
missed adding a virObject field to the virHostdevManager struct.
Result is memory corruption when taking a reference on an instance
of the object, where atomic inc is done on the stateDir field.
Later use of stateDir crashes libvirtd.