libxl: Fix handling of timeouts

libxl: Fix handling of timeouts

xen-unstable changeset 26469 makes changes wrt modifying and deregistering
timeouts.

First, timeout modify callbacks will only be invoked with an
abs_t of {0,0}, i.e. make the timeout fire immediately.  Prior to this
commit, timeout modify callbacks were never invoked.

Second, timeout deregister hooks will no longer be called.

This patch makes changes in the libvirt libxl driver that should be
compatible before and after changeset 26469.

While at it, fix a potential overflow in the timeout register callback.