libxl: Fix race between destruction of objects

libxl: Fix race between destruction of objects

It is possible to destroy and cleanup a VM, resulting in freeing the
libxlDomainObjPrivate object and associated libxl ctx, before all fds and
timeouts have been deregistered and destroyed.

Fix this race by incrementing the reference count on libxlDomainObjPrivate
for each fd and timeout registration.  Only when all fds and timeouts are
deregistered and destroyed will the libxlDomainObjPrivate be destroyed.