When creating a timer/event handler reference counting is used. So it could
be possible (in theory) that libxlDomainObjPrivateFree is called with
reference counting >1. The problem is that libxlDomainObjPrivateFree leave
the object in an invalid state with ctx freed (but still having dandling
pointer). This can lead timer/event handler to core.

This patch implements a dispose method for libxlDomainObjPrivate, and moves
freeing the libxl ctx to the dispose method, ensuring the ctx is valid while
the object's reference count is > 0.
Signed-off-by: NFrediano Ziglio <frediano.ziglio@citrix.com>