libxl: Use id from virDomainObj inside the driver

libxl: Use id from virDomainObj inside the driver

There is a domain id in the virDomain structure as well as in the
virDomainObj structure. While the former can become stale the latter
is kept up to date. So it is safer to always (virDomainObjPtr)->def->id
internally.

This will fix issues seen when managing Xen guests through libvirt from
virt-manager (not being able to get domain info after define or reboot).
This was caused both though libxlDomainGetInfo() only but there were
a lot of places that might potentially cause issues, too.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>