Convert libxl driver to Xen 4.2

Convert libxl driver to Xen 4.2

Based on a patch originally authored by Daniel De Graaf

  http://lists.xen.org/archives/html/xen-devel/2012-05/msg00565.html

This patch converts the Xen libxl driver to support only Xen >= 4.2.
Support for Xen 4.1 libxl is dropped since that version of libxl is
designated 'technology preview' only and is incompatible with Xen 4.2
libxl.  Additionally, the default toolstack in Xen 4.1 is still xend,
for which libvirt has a stable, functional driver.