libxl: Fix races in libxl event code

libxl: Fix races in libxl event code

The libxl driver is racy in it's interactions with libxl and libvirt's
event loop.  The event loop can invoke callbacks after libxl has
deregistered the event, and possibly access freed data associated with
the event.

This patch fixes the race by converting libxlDomainObjPrivate to a
virObjectLockable, and locking it while executing libxl upcalls and
libvirt event loop callbacks.

Note that using the virDomainObj lock is not satisfactory since it may
be desirable to hold the virDomainObj lock even when libxl events such
as reading and writing to xenstore need processed.